Build ramdisks in an image chroot.

Ramdisks are now built inside a chroot which is built by the normal
image build process. Doing so improves our independence of the
precise state of the build host.

This fixes bug 1194055.

Change-Id: Ibc254fbb9e7b404b5f38c1b35bcde8a4136e8e28
Merge "Build ramdisks in an image chroot."