rootfs: Allow caching of the raw rootfs

In flaky network environments, rebuilding the rootfs raw image becomes a
significant pain and in some cases, nearly impossible. This change allows us to
bypass the need to regenerate the raw rootfs every build and instead use a
rootfs placed in the cache directory.

Change-Id: Iaf0476ce835a379f2fdd69906f0cb3c0a190635d
1 file changed