cleanup: remove a couple references to chrome-bot
cleanup: remove source location migration code

This code is not applicable to us, it predates CoreOS and is a weird
thing for common.sh to be doing as well. Instead always define
CHROOT_TRUNK_DIR to /mnt/host/source, create ~/trunk in make_chroot.
make_chroot: remove unneeded modifications

These are assorted tweaks that are already setup in the tarball.
Merge pull request #331 from marineam/cleanup

Misc cleanup