rp2,esp32,extmod: Implement UPDATE_SUBMODULES in CMake.

Rather than having Make calling CMake to generate a list of submodules and
then run a Make target (which is complex and prone to masking other
errors), implement the submodule update logic in CMake itself.

Internal CMake-side changes are that GIT_SUBMODULES is now a CMake list,
and the trigger variable name is changed from ECHO_SUBMODULES to
UPDATE_SUBMODULES.

The run is otherwise 100% a normal CMake run now, so most of the other
special casing can be removed.

Signed-off-by: Angus Gratton <angus@redyak.com.au>
diff --git a/ports/esp32/Makefile b/ports/esp32/Makefile
index 1ce4d97..a4e5531 100644
--- a/ports/esp32/Makefile
+++ b/ports/esp32/Makefile
@@ -106,12 +106,10 @@
 size-files:
 	$(call RUN_IDF_PY,size-files)
 
-# Running the build with ECHO_SUBMODULES set will trigger py/mkrules.cmake to
-# print out the value of the GIT_SUBMODULES variable, prefixed with
-# "GIT_SUBMODULES", and then abort. This extracts out that line from the idf.py
-# output and passes the list of submodules to py/mkrules.mk which does the
-# `git submodule init` on each.
+# Run idf.py with the UPDATE_SUBMODULES flag to update
+# necessary submodules for this board.
+#
+# This is done in a dedicated build directory as some CMake cache values are not
+# set correctly if not all submodules are loaded yet.
 submodules:
-	@GIT_SUBMODULES=$$(IDF_COMPONENT_MANAGER=0 idf.py $(IDFPY_FLAGS) -B $(BUILD)/submodules -D ECHO_SUBMODULES=1 build 2>&1 | \
-	                  grep '^GIT_SUBMODULES=' | cut -d= -f2); \
-	$(MAKE) -f ../../py/mkrules.mk GIT_SUBMODULES="$${GIT_SUBMODULES}" GIT_SUBMODULES_FAIL_IF_EMPTY=1 submodules
+	IDF_COMPONENT_MANAGER=0 idf.py $(IDFPY_FLAGS) -B $(BUILD)/submodules -D UPDATE_SUBMODULES=1 reconfigure
diff --git a/ports/esp32/esp32_common.cmake b/ports/esp32/esp32_common.cmake
index 6473f04..f7b0090 100644
--- a/ports/esp32/esp32_common.cmake
+++ b/ports/esp32/esp32_common.cmake
@@ -71,8 +71,8 @@
     ${MICROPY_DIR}/drivers/dht/dht.c
 )
 
-string(CONCAT GIT_SUBMODULES "${GIT_SUBMODULES} " lib/tinyusb)
-if(MICROPY_PY_TINYUSB AND NOT ECHO_SUBMODULES)
+list(APPEND GIT_SUBMODULES lib/tinyusb)
+if(MICROPY_PY_TINYUSB)
     set(TINYUSB_SRC "${MICROPY_DIR}/lib/tinyusb/src")
     string(TOUPPER OPT_MCU_${IDF_TARGET} tusb_mcu)
 
@@ -195,6 +195,17 @@
     set(MICROPY_LDFRAGMENTS ${MICROPY_USER_LDFRAGMENTS})
 endif()
 
+if (UPDATE_SUBMODULES)
+    # ESP-IDF checks if some paths exist before CMake does. Some paths don't
+    # yet exist if this is an UPDATE_SUBMODULES pass on a brand new checkout, so remove
+    # any path which might not exist yet. A "real" build will not set UPDATE_SUBMODULES.
+    unset(MICROPY_SOURCE_TINYUSB)
+    unset(MICROPY_SOURCE_EXTMOD)
+    unset(MICROPY_SOURCE_LIB)
+    unset(MICROPY_INC_TINYUSB)
+    unset(MICROPY_INC_CORE)
+endif()
+
 # Register the main IDF component.
 idf_component_register(
     SRCS
diff --git a/ports/rp2/CMakeLists.txt b/ports/rp2/CMakeLists.txt
index 7002ad8..f89e279 100644
--- a/ports/rp2/CMakeLists.txt
+++ b/ports/rp2/CMakeLists.txt
@@ -76,8 +76,8 @@
 endif()
 
 # Necessary submodules for all boards.
-string(CONCAT GIT_SUBMODULES "${GIT_SUBMODULES} " lib/mbedtls)
-string(CONCAT GIT_SUBMODULES "${GIT_SUBMODULES} " lib/tinyusb)
+list(APPEND GIT_SUBMODULES lib/mbedtls)
+list(APPEND GIT_SUBMODULES lib/tinyusb)
 
 # Include component cmake fragments
 include(${MICROPY_DIR}/py/py.cmake)
@@ -340,7 +340,7 @@
 endif()
 
 if (MICROPY_BLUETOOTH_BTSTACK)
-    string(CONCAT GIT_SUBMODULES "${GIT_SUBMODULES} " lib/btstack)
+    list(APPEND GIT_SUBMODULES lib/btstack)
 
     list(APPEND MICROPY_SOURCE_PORT mpbtstackport.c)
 
@@ -358,8 +358,8 @@
 endif()
 
 if(MICROPY_BLUETOOTH_NIMBLE)
-    string(CONCAT GIT_SUBMODULES "${GIT_SUBMODULES} " lib/mynewt-nimble)
-    if(NOT (${ECHO_SUBMODULES}) AND NOT EXISTS ${MICROPY_DIR}/lib/mynewt-nimble/nimble/host/include/host/ble_hs.h)
+    list(APPEND GIT_SUBMODULES lib/mynewt-nimble)
+    if(NOT UPDATE_SUBMODULES AND NOT EXISTS ${MICROPY_DIR}/lib/mynewt-nimble/nimble/host/include/host/ble_hs.h)
         message(FATAL_ERROR " mynewt-nimble not initialized.\n Run 'make BOARD=${MICROPY_BOARD} submodules'")
     endif()
 
@@ -386,8 +386,8 @@
 )
 
 if (MICROPY_PY_NETWORK_CYW43)
-    string(CONCAT GIT_SUBMODULES "${GIT_SUBMODULES} " lib/cyw43-driver)
-    if((NOT (${ECHO_SUBMODULES})) AND NOT EXISTS ${MICROPY_DIR}/lib/cyw43-driver/src/cyw43.h)
+    list(APPEND GIT_SUBMODULES lib/cyw43-driver)
+    if(NOT UPDATE_SUBMODULES AND NOT EXISTS ${MICROPY_DIR}/lib/cyw43-driver/src/cyw43.h)
         message(FATAL_ERROR " cyw43-driver not initialized.\n Run 'make BOARD=${MICROPY_BOARD} submodules'")
     endif()
 
@@ -433,8 +433,8 @@
 endif()
 
 if (MICROPY_PY_NETWORK_WIZNET5K)
-    string(CONCAT GIT_SUBMODULES "${GIT_SUBMODULES} " lib/wiznet5k)
-    if((NOT (${ECHO_SUBMODULES})) AND NOT EXISTS ${MICROPY_DIR}/lib/wiznet5k/README.md)
+    list(APPEND GIT_SUBMODULES lib/wiznet5k)
+    if(NOT UPDATE_SUBMODULES AND NOT EXISTS ${MICROPY_DIR}/lib/wiznet5k/README.md)
         message(FATAL_ERROR " wiznet5k not initialized.\n Run 'make BOARD=${MICROPY_BOARD} submodules'")
     endif()
 
diff --git a/ports/rp2/Makefile b/ports/rp2/Makefile
index 200899d..bfc85f3 100644
--- a/ports/rp2/Makefile
+++ b/ports/rp2/Makefile
@@ -65,15 +65,11 @@
 clean:
 	$(RM) -rf $(BUILD)
 
-# First ensure that pico-sdk is initialised, then use cmake to pick everything
-# else (including board-specific dependencies).
-# Running the build with ECHO_SUBMODULES set will trigger py/mkrules.cmake to
-# print out the value of the GIT_SUBMODULES variable, prefixed with
-# "GIT_SUBMODULES", and then abort. This extracts out that line from the cmake
-# output and passes the list of submodules to py/mkrules.mk which does the
-# `git submodule init` on each.
+# First ensure that pico-sdk is initialised, then run CMake with the
+# UPDATE_SUBMODULES flag to update necessary submodules for this board.
+#
+# This is done in a dedicated build directory as some CMake cache values are not
+# set correctly if not all submodules are loaded yet.
 submodules:
 	$(MAKE) -f ../../py/mkrules.mk GIT_SUBMODULES="lib/pico-sdk" submodules
-	@GIT_SUBMODULES=$$(cmake -B $(BUILD)/submodules -DECHO_SUBMODULES=1 ${CMAKE_ARGS} -S . 2>&1 | \
-	                  grep '^GIT_SUBMODULES=' | cut -d= -f2); \
-	$(MAKE) -f ../../py/mkrules.mk GIT_SUBMODULES="$${GIT_SUBMODULES}" GIT_SUBMODULES_FAIL_IF_EMPTY=1 submodules
+	cmake -S . -B $(BUILD)/submodules -DUPDATE_SUBMODULES=1 ${CMAKE_ARGS}