core: Fix chibios user compile options

Compile options can be defined in project Makefile such as UDEFS, UADEFS, UINCDIR, ULIBDIR and ULIBS.

tmk_core/tool/chibios/chibios.mk

@@ -201,10 +201,10 @@ CPPWARN = -Wall -Wextra -Wundef
 
 # List all user C define here, like -D_DEBUG=1
 ## Select which interfaces to include here!
-UDEFS = $(OPT_DEFS)
+UDEFS += $(OPT_DEFS)
 
 # Define ASM defines here
-UADEFS = $(OPT_DEFS)
+UADEFS += $(OPT_DEFS)
 # bootloader definitions may be used in the startup .s file
 ifneq ("$(wildcard $(TARGET_DIR)/bootloader_defs.h)","")
     UADEFS += -include $(TARGET_DIR)/bootloader_defs.h
@@ -215,13 +215,13 @@ else ifneq ("$(wildcard $(TARGET_DIR)/boards/$(BOARD)/bootloader_defs.h)","")
 endif
 
 # List all user directories here
-UINCDIR =
+#UINCDIR =
 
 # List the user directory to look for the libraries here
-ULIBDIR =
+#ULIBDIR =
 
 # List all user libraries here
-ULIBS =
+#ULIBS =
 
 #
 # End of user defines