fix(build_library/legacy_disk_layout): Use coreos- prefixes

vboot_reference now recognizes coreos-reserved and coreos-rootfs. Use
these prefixes so we stop using the chromeos GUIDs.

Test-plan: Tested on a VM and it boots and updates.
Merge pull request #24 from philips/use-coreos-prefix

fix(build_library/legacy_disk_layout): Use coreos- prefixes