Merge pull request contiki-os#418 from frtos-wpan/uninterrupted-cleanup

Makefile.include (clean): ignore rm -f failing (e.g., on "core")

Makefile.include

@@ -192,7 +192,7 @@ rm -f $(@:.o=.$$$$)
 endef
 
 clean:
-	rm -f *~ *core core *.srec \
+	-rm -f *~ *core core *.srec \
 	*.lst *.map \
 	*.cprg *.bin *.data contiki*.a *.firmware core-labels.S *.ihex *.ini \
 	*.ce *.co