stm32/Makefile: Allow QSTR_DEFS,QSTR_GLOBAL_DEPENDENCIES to be extended.

So a board can provide custom qstr definitions if needed.
Signed-off-by: Damien George <damien@micropython.org>

ports/stm32/Makefile

@@ -18,8 +18,8 @@ include $(BOARD_DIR)/mpconfigboard.mk
 QSTR_GENERATED_HEADERS = $(BUILD)/pins_qstr.h $(BUILD)/modstm_qstr.h
 
 # qstr definitions (must come before including py.mk)
-QSTR_DEFS = qstrdefsport.h $(QSTR_GENERATED_HEADERS)
-QSTR_GLOBAL_DEPENDENCIES = mpconfigboard_common.h $(BOARD_DIR)/mpconfigboard.h $(QSTR_GENERATED_HEADERS)
+QSTR_DEFS += qstrdefsport.h $(QSTR_GENERATED_HEADERS)
+QSTR_GLOBAL_DEPENDENCIES += mpconfigboard_common.h $(BOARD_DIR)/mpconfigboard.h $(QSTR_GENERATED_HEADERS)
 
 # MicroPython feature configurations
 MICROPY_ROM_TEXT_COMPRESSION ?= 1