Makefile.include: Set PREFIX if not set

Makefile.include

@@ -159,6 +159,10 @@ endif
 
 export TOOLCHAIN
 
+# default toolchain prefix, defaults to target triple followed by a dash, you
+# will most likely not need to touch this.
+export PREFIX ?= $(if $(TARGET_ARCH),$(TARGET_ARCH)-)
+
 # Import all toolchain settings
 include $(RIOTCPU)/Makefile.include.$(TOOLCHAIN)
 

cpu/Makefile.include.cortexm_common

@@ -1,10 +1,6 @@
 # Target triple for the build. Use arm-none-eabi if you are unsure.
 export TARGET_ARCH ?= arm-none-eabi
 
-# default toolchain prefix, defaults to target triple followed by a dash, you
-# will most likely not need to touch this.
-export PREFIX ?= $(if $(TARGET_ARCH),$(TARGET_ARCH)-)
-
 # define build specific options
 export CFLAGS_CPU   = -mcpu=$(MCPU) -mlittle-endian -mthumb $(CFLAGS_FPU)
 ifneq (llvm,$(TOOLCHAIN))