py/mkrules.cmake: Add MICROPY_QSTRDEFS_PORT to qstr build process.

This allows a port to specify a custom qstrdefsport.h file, the same as the
QSTR_DEFS variable in a Makefile.

Signed-off-by: Damien George <damien@micropython.org>
diff --git a/py/mkrules.cmake b/py/mkrules.cmake
index 304f2be..9c4c4af 100644
--- a/py/mkrules.cmake
+++ b/py/mkrules.cmake
@@ -95,8 +95,10 @@
 
 add_custom_command(
     OUTPUT ${MICROPY_QSTRDEFS_PREPROCESSED}
-    COMMAND cat ${MICROPY_QSTRDEFS_PY} ${MICROPY_QSTRDEFS_COLLECTED} | sed "s/^Q(.*)/\"&\"/" | ${CMAKE_C_COMPILER} -E ${MICROPY_CPP_FLAGS} - | sed "s/^\\\"\\(Q(.*)\\)\\\"/\\1/" > ${MICROPY_QSTRDEFS_PREPROCESSED}
-    DEPENDS ${MICROPY_QSTRDEFS_COLLECTED}
+    COMMAND cat ${MICROPY_QSTRDEFS_PY} ${MICROPY_QSTRDEFS_PORT} ${MICROPY_QSTRDEFS_COLLECTED} | sed "s/^Q(.*)/\"&\"/" | ${CMAKE_C_COMPILER} -E ${MICROPY_CPP_FLAGS} - | sed "s/^\\\"\\(Q(.*)\\)\\\"/\\1/" > ${MICROPY_QSTRDEFS_PREPROCESSED}
+    DEPENDS ${MICROPY_QSTRDEFS_PY}
+        ${MICROPY_QSTRDEFS_PORT}
+        ${MICROPY_QSTRDEFS_COLLECTED}
     VERBATIM
     COMMAND_EXPAND_LISTS
 )