Avoid luneos assumptions in initramfs (issue #621)

init-tenderloin.sh will now make use of variables in distro.conf
to achieve the desired behavior, in order to avoid luneos-specific
assumptions in the script itself. Distro-specific assumptions are
best left to distro-specific layers such as meta-webos-ports,
where the luneos distro.conf resides.

The relevant variables are distro_name (already exists) and
distro_rootfs_file (new). The first is used in generating a
volume name; the latter to sanity check whether a rootfs is
valid or the fallback should be tried. If distro_rootfs_file
is left unset, then the sanity check automatically passes.

For equivalent behavior to commit c4a46727dc, luneos distro.conf
should include:

distro_name=luneos
distro_rootfs_file=/etc/webos-release

This will result in trying volumes "luneos-root" then "ext3fs",
picking the first one both mountable and containing /etc/webos-release,
or giving up on boot if both fail the criteria.

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