scripts/combo-layer: use last_revision if specified in init
If last_revision is specified for a component when running combo-layer
init, then use that revision instead of the latest revision on the
branch. Also, remove unnecessary git checkout during init since we
specify the revision to all calls to git when dealing with the component
repositories.
Fixes [YOCTO #3040].
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Saul Wold <sgw@linux.intel.com>
