initramfs checks 2 places for rootfs (issue #621)

http://issues.webos-ports.org/issues/621

When installing LuneOS, the generic volume "ext3fs" may already contain
something else, so there should be an alternate install procedure allowing
use of a more uniquely named volume, i.e. "luneos-root".

Prepare for such installs by having the initramfs script check first
for "luneos-root" and then fall back to "ext3fs" when trying to mount root.
Each candidate root is sanity-checked by looking for existence of
/etc/webos-release before picking it for switchroot.

Signed-off-by: S. Lockwood-Childs <sjl@vctlabs.com>