LUKS: fix bootonroot

Teej Tjrequested to merge
Iam_Tj/mobian-recipes:fix-luks_bootonroot_issue54 into master

When using LUKS with bootonroot TARGET_DISK is not set resulting in script failure.

Handle the case where the only mount is the ROOTDIR.

Fixes: issue #54 Signed-off-by: Tj debian@iam.tj

Merge request reports