rp2/CMakeLists.txt: Allow a board's cmake to set the manifest path.

This allows boards to add frozen modules, or bypass the port manifest
entirely.

ports/rp2/CMakeLists.txt

@@ -163,7 +163,9 @@ set(PICO_SDK_COMPONENTS
 
 # Define mpy-cross flags and frozen manifest
 set(MICROPY_CROSS_FLAGS -march=armv7m)
-set(MICROPY_FROZEN_MANIFEST ${PROJECT_SOURCE_DIR}/boards/manifest.py)
+if (NOT MICROPY_FROZEN_MANIFEST)
+    set(MICROPY_FROZEN_MANIFEST ${PROJECT_SOURCE_DIR}/boards/manifest.py)
+endif()
 
 target_sources(${MICROPY_TARGET} PRIVATE
     ${MICROPY_SOURCE_PY}