atmel-samd: enable build error for implicit fallthrough

If any diagnostics occur, we will want to either add `/* FALLTHROUGH */`
or `break;` as appropriate.  I only tested a few builds (trinket m0
and metro m4 express)

Author: jepler

ports/atmel-samd/Makefile

@@ -148,7 +148,7 @@ else
   endif
 endif
 
-CFLAGS += $(INC) -Wall -Werror -std=gnu11 -nostdlib -fshort-enums $(BASE_CFLAGS) $(CFLAGS_MOD) $(COPT)
+CFLAGS += $(INC) -Wall -Werror -std=gnu11 -nostdlib -fshort-enums $(BASE_CFLAGS) $(CFLAGS_MOD) $(COPT) -Wimplicit-fallthrough=2
 
 ifeq ($(CHIP_FAMILY), samd21)
 CFLAGS += \