rootfs make target

New makefile target to create rootfs.vhd from rootfs.tar using tar2ext4.
Requires building cmd/tar2ext4 locally.

Signed-off-by: Hamza El-Saawy <hamzaelsaawy@microsoft.com>

Makefile

@@ -23,6 +23,8 @@ GCS_TOOLS=\
 
 .PHONY: all always rootfs test
 
+.DEFAULT_GOAL := all
+
 all: out/initrd.img out/rootfs.tar.gz
 
 clean:
@@ -32,6 +34,12 @@ clean:
 test:
  cd $(SRCROOT) && go test -v ./internal/guest/...
 
+rootfs: out/rootfs.vhd
+
+out/rootfs.vhd: out/rootfs.tar.gz bin/cmd/tar2ext4
+ gzip -f -d ./out/rootfs.tar.gz
+ bin/cmd/tar2ext4 -vhd -i ./out/rootfs.tar -o $@
+
 out/delta.tar.gz: bin/init bin/vsockexec bin/cmd/gcs bin/cmd/gcstools Makefile
  @mkdir -p out
  rm -rf rootfs
@@ -60,6 +68,7 @@ out/initrd.img: $(BASE) out/delta.tar.gz $(SRCROOT)/hack/catcpio.sh
 
 -include deps/cmd/gcs.gomake
 -include deps/cmd/gcstools.gomake
+-include deps/cmd/tar2ext4.gomake
 
 # Implicit rule for includes that define Go targets.
 %.gomake: $(SRCROOT)/Makefile