fix(build_image): Add support for COREOS_DISK_LAYOUT_SUFFIX

Append COREOS_DISK_LAYOUT_SUFFIX from the portage profile to disk layout
names. Allows switching from "base" to "base-usr" and similar.
fix(create_legacy_bootloader_templates.sh): Support -usr layouts

The boot kernel parameters change depending on whether the new /usr
scheme is in use. Pass the disk layout to the bootloader config script
and adjust generated configs accordingly.
fix(build_library): Add amd64-usr board and disk layouts.

The basic infrastructure to support this is now in place. Add a new
board that uses the experimental coreos/amd64/usr profile /usr based
disk layouts. This is just enough to successfully build images, they
aren't bootable yet.
Merge pull request #148 from marineam/switch-usr

Experimantal amd64-usr board and disk layouts