Enable custom chibios sub-platform

tmk_core/chibios.mk

@@ -39,9 +39,13 @@ include $(STARTUP_MK)
 # HAL-OSAL files (optional).
 include $(CHIBIOS)/os/hal/hal.mk
 
-PLATFORM_MK = $(CHIBIOS)/os/hal/ports/$(MCU_FAMILY)/$(MCU_SERIES)/platform.mk
+ifeq ("$(PLATFORM_NAME)","")
+	PLATFORM_NAME = platform
+endif
+
+PLATFORM_MK = $(CHIBIOS)/os/hal/ports/$(MCU_FAMILY)/$(MCU_SERIES)/$(PLATFORM_NAME).mk
 ifeq ("$(wildcard $(PLATFORM_MK))","")
-PLATFORM_MK = $(CHIBIOS_CONTRIB)/os/hal/ports/$(MCU_FAMILY)/$(MCU_SERIES)/platform.mk
+PLATFORM_MK = $(CHIBIOS_CONTRIB)/os/hal/ports/$(MCU_FAMILY)/$(MCU_SERIES)/$(PLATFORM_NAME).mk
 endif
 include $(PLATFORM_MK)