rootfs: Add make install target to install files for NFS server

rootfs/build/GNUmakefile

@@ -10,7 +10,7 @@ MAKEFLAGS += --no-print-directory
 
 .PHONY: all $(MAKECMDGOALS)
 
-all	:= $(filter-out all Makefile,$(MAKECMDGOALS))
+all	:= $(filter-out all Makefile install,$(MAKECMDGOALS))
 
 all:
 	$(MAKE) $(MAKEARGS) $(all)
@@ -22,3 +22,7 @@ $(all): all
 
 %/: all
 	@:
+
+install:
+	install -d -m 755 $(DESTDIR)/srv/nfs/ryu-rootfs
+	tar -C $(DESTDIR)/srv/nfs/ryu-rootfs -xf images/rootfs.tar