Fix missing rootfs Makefile target

The README indicated that download and compling process can be done with
`make rootfs` command. However, the corresponding Makefile target was
removed in commit 25571f7: Modularize the build system.

Originally, the rootfs target was introduced in commit 9d5eac5: Parse the
cmdline arguments more perfectly, but was later removed in the aforement-
ioned commit without updates to the README or integration into
mk/external.mk

Makefile

@@ -66,6 +66,8 @@ $(OUT)/ext4.img:
 	$(Q)dd if=/dev/zero of=$@ bs=4k count=600
 	$(Q)mkfs.ext4 -F $@
 
+rootfs: $(ROOTFS_IMG)
+
 check: $(BIN) $(LINUX_IMG) $(ROOTFS_IMG) $(OUT)/ext4.img
 	$(VECHO) "\nOnce the message 'Kernel panic' appears, press Ctrl-C to exit\n\n"
 	$(Q)sudo $(BIN) -k $(LINUX_IMG) -i $(ROOTFS_IMG) -d $(OUT)/ext4.img
@@ -75,6 +77,6 @@ clean:
 	$(Q)rm -f $(OBJS) $(deps) $(BIN)
 
 distclean: clean
-	$(Q)rm -rf build
+	$(Q)rm -rf $(OUT)
 
 -include $(deps)