Vagrant reads this file to determine that we are CoreOS... so lets not
break that just yet. A PR to switch to os-release has been posted:
https://github.com/mitchellh/vagrant/pull/2985
Some day gentoo-release will be dropped but that day is not today.
Check if the disk layout is a /usr layout and if so hack the USR-A
partition, not whatever is mounted to /. Also use the new functionality
in disk_util for this as it can look up partitions by label.
cros_make_image_bootable now only is relevant for prod images, so move
the remaining code to prod_image_util in a similar scheme that base and
dev images use.
