Create the sfs root directory if missing

Signed-off-by: Junjie Mao <junjie.mao@hotmail.com>

labcodes/lab8/Makefile

@@ -273,6 +273,9 @@ $(foreach p,$(USER_BINS),$(eval $(call fscopy,$(p),$(SFSROOT)$(SLASH))))
 $(SFSROOT):
 	if [ ! -d "$(SFSROOT)" ]; then mkdir $(SFSROOT); fi
 
+$(SFSROOT):
+	$(V)$(MKDIR) $@
+
 $(SFSIMG): $(SFSROOT) $(SFSBINS) | $(call totarget,mksfs)
 	$(V)dd if=/dev/zero of=$@ bs=1M count=128
 	@$(call totarget,mksfs) $@ $(SFSROOT)