nrf/Makefile: use "standard" GCC -fshort-enums instead of --short-enums.

Clang understands only -fshort-enums, not --short-enums. As
--short-enums isn't even mentioned in the gcc man page, I think this
alias exists more for backwards compatibility.

ports/nrf/Makefile

@@ -89,7 +89,7 @@ CFLAGS_CORTEX_M = -mthumb -mabi=aapcs -fsingle-precision-constant -Wdouble-promo
 
 CFLAGS_MCU_m4 = $(CFLAGS_CORTEX_M) -mtune=cortex-m4 -mcpu=cortex-m4 -mfpu=fpv4-sp-d16 -mfloat-abi=hard
 
-CFLAGS_MCU_m0 = $(CFLAGS_CORTEX_M) --short-enums -mtune=cortex-m0 -mcpu=cortex-m0 -mfloat-abi=soft -fno-builtin
+CFLAGS_MCU_m0 = $(CFLAGS_CORTEX_M) -fshort-enums -mtune=cortex-m0 -mcpu=cortex-m0 -mfloat-abi=soft -fno-builtin
 
 LTO ?= 1
 ifeq ($(LTO),1)