Treat linker warnings as errors

When using relative path (ie `./contiki-ng`) some Makefiles, like cortex m3, fails
to filter out ldscripts resulting in a broken build.

Makefile.include

@@ -49,6 +49,8 @@ CFLAGS += -DCONTIKI_BOARD_$(TARGET_BOARD_UPPERCASE)=1
 CFLAGS += -DCONTIKI_BOARD_STRING=\"$(BOARD)\"
 endif
 
+LDFLAGS = -Wl,--fatal-warnings
+
 MODULES += os os/sys os/dev os/lib os/services
 
 # Automatically include project-conf.h if found