chore(*): use COREOS lsb-release variables

update these ebuilds to use the COREOS lsb-release variables

coreos-installer/coreos-installer
cros-devutils/cros-devutils
gmerge/gmerge
update_engine/update_engine
vboot_reference/vboot_reference

Tested by building an image and checking gmerge and update_engine still work successfully with a cleaned up lsb-release.