Synchronize localtime between Chrome OS and host OS:
 - bindmount /var/lib/timezone
 - During installation, link /etc/localtime to /var/lib/timezone/localtime
Merge remote-tracking branch 'mycrouton/localtime'
- Update arch post-common to create localtime symlink.
- Revert past modification that used to copy over localtime.
Enable bootstrapping of Arch Linux.