cortexm: Make linker warnings fatal

Linker warnings usually means something does not get placed where it is
supposed to and the most likely result is a broken binary.
Linker warnings should be treated as serious problems with the program
and should be solved before the program is tested on actual hardware.

boards/Makefile.include.cortexm_common

@@ -16,7 +16,7 @@ export CFLAGS_OPT   ?= -Os
 export CFLAGS += $(CFLAGS_CPU) $(CFLAGS_STYLE) $(CFLAGS_LINK) $(CFLAGS_DBG) $(CFLAGS_OPT)
 
 export ASFLAGS += $(CFLAGS_CPU) $(CFLAGS_DEBUG)
-export LINKFLAGS += -L$(RIOTCPU)/$(CPU)/ldscripts -T$(RIOTCPU)/$(CPU)/ldscripts/$(CPU_MODEL).ld
+export LINKFLAGS += -L$(RIOTCPU)/$(CPU)/ldscripts -T$(RIOTCPU)/$(CPU)/ldscripts/$(CPU_MODEL).ld -Wl,--fatal-warnings
 export LINKFLAGS += $(CFLAGS_DEBUG) $(CFLAGS_CPU) $(CFLAGS_STYLE) -static -lgcc -nostartfiles
 
 # use the nano-specs of the NewLib when available