Fix some funky dependencies around rootfs images

- Doing m followed by m sdcard would cause lots of stuff to happen
twice, due to refetching the raw rootfs. Also generally refeching the
root tarball was happening way more than it ever should.
- Clean up the way fetching the rootfs happens,
such that it's not re-copied unless actually changed.

Change-Id: I8af7b144c0f659482b2b7ddef318dce6a54cdeaa
1 file changed