cpu: cortexm_common: allow overriding of linker script

cpu/Makefile.include.cortexm_common

@@ -15,8 +15,9 @@ export CFLAGS_OPT  ?= -Os
 export CFLAGS += $(CFLAGS_CPU) $(CFLAGS_LINK) $(CFLAGS_DBG) $(CFLAGS_OPT)
 
 export ASFLAGS += $(CFLAGS_CPU) $(CFLAGS_DBG)
+export LINKER_SCRIPT ?= $(RIOTCPU)/$(CPU)/ldscripts/$(CPU_MODEL).ld
 export LINKFLAGS += -L$(RIOTCPU)/$(CPU)/ldscripts -L$(RIOTCPU)/cortexm_common/ldscripts
-export LINKFLAGS += -T$(RIOTCPU)/$(CPU)/ldscripts/$(CPU_MODEL).ld -Wl,--fatal-warnings
+export LINKFLAGS += -T$(LINKER_SCRIPT) -Wl,--fatal-warnings
 export LINKFLAGS += $(CFLAGS_CPU) $(CFLAGS_DBG) $(CFLAGS_OPT) -static -lgcc -nostartfiles
 export LINKFLAGS += -Wl,--gc-sections