Use arch-specific rootfs

- boot and rootfs can now be built for multiple architectures without
conflicting with each other.
- Kokoro generates arm64 and armhf versions of the mmc image, root
filesystems, and sdcard image. It also generates a recovery sdcard.
- Tighten up the partition-table %.img wildcard.

Change-Id: Ieb9a073bf4b94af7f731f3685c457e9c020b0c83
7 files changed