switch to luneos as distro_name in initramfs

initramfs script uses distro_name to find rootfs directories (or rootfs volume
in tenderloin case). Project has been renamed, and directory names/volumes
should use the new name.

Also adding a new variable distro_rootfs_file which has the name of a file
which is known to exist in a luneos release. distro_rootfs_file will be used
to sanity-check a prospective rootfs volume in the tenderloin-specific
initramfs script (see https://github.com/webOS-ports/meta-smartphone/pull/7)

Despite webos being replaced in distro_name, distro_rootfs_file points to
/etc/webos-release because there "webos" represents the family of distros,
like when debian-based distros still create /etc/debian_version

Signed-off-by: S. Lockwood-Childs <sjl@vctlabs.com>