py/py.cmake: Introduce MICROPY_INC_CORE as a list with core includes.

Signed-off-by: Damien George <damien@micropython.org>
diff --git a/ports/esp32/main/CMakeLists.txt b/ports/esp32/main/CMakeLists.txt
index 1f4bb31..2bfc095 100644
--- a/ports/esp32/main/CMakeLists.txt
+++ b/ports/esp32/main/CMakeLists.txt
@@ -132,7 +132,7 @@
         ${MICROPY_SOURCE_DRIVERS}
         ${MICROPY_SOURCE_PORT}
     INCLUDE_DIRS
-        ${MICROPY_DIR}
+        ${MICROPY_INC_CORE}
         ${MICROPY_INC_USERMOD}
         ${MICROPY_PORT_DIR}
         ${MICROPY_BOARD_DIR}