Use CROSS_COMPILE variable to select toolchain

This makes it possible to select the build toolchain without modifying
the Makefile.

Makefile

@@ -4,9 +4,10 @@ SRCS	= main.c system_clock.c system_interrupts.c status_led.c usb_core.c usb_des
 	usb_io.c usb_uid.c usb_panic.c usb_cdc.c cdc_shell.c gpio.c device_config.c
 
 # Toolchain & Utils
-CC		= arm-none-eabi-gcc
-OBJCOPY		= arm-none-eabi-objcopy
-SIZE		= arm-none-eabi-size
+CROSS_COMPILE	?= arm-none-eabi-
+CC		= $(CROSS_COMPILE)gcc
+OBJCOPY		= $(CROSS_COMPILE)objcopy
+SIZE		= $(CROSS_COMPILE)size
 STFLASH		= st-flash
 STUTIL		= st-util
 CPPCHECK	= cppcheck