py/mkrules.cmake: Clean genhdr and frozen_mpy dirs.

CMake builds relied upon the parent Makefile removing the entire
build directory to successfully clean build artifacts.

py/mkrules.cmake: Add ADDITIONAL_CLEAN_FILES properties to ensure a
		  "make clean" from within the build directory removes
                  the genhdr and frozen_mpy directories.

Signed-off-by: Phil Howard <github@gadgetoid.com>

py/mkrules.cmake

@@ -100,6 +100,12 @@ if(MICROPY_ROM_TEXT_COMPRESSION)
     )
 endif()
 
+# Ensure genhdr directory is removed on clean
+
+set_property(TARGET ${MICROPY_TARGET} APPEND PROPERTY ADDITIONAL_CLEAN_FILES
+    "${MICROPY_GENHDR_DIR}"
+)
+
 # Command to force the build of another command
 
 # Generate mpversion.h
@@ -244,6 +250,10 @@ add_custom_command(
 if(MICROPY_FROZEN_MANIFEST)
     set(MICROPY_FROZEN_CONTENT "${CMAKE_BINARY_DIR}/frozen_content.c")
 
+    set_property(TARGET ${MICROPY_TARGET} APPEND PROPERTY ADDITIONAL_CLEAN_FILES
+        "${CMAKE_BINARY_DIR}/frozen_mpy"
+    )
+
     target_sources(${MICROPY_TARGET} PRIVATE
         ${MICROPY_FROZEN_CONTENT}
     )