scripts/combo-layer: ensure we validate branch/revision on init

scripts/combo-layer: ensure we validate branch/revision on init

If both branch and last_revision are specified for a component when
combo-layer init is run, ensure that the specified revision is actually
on the specified branch and error out if not. Also ensure that the error
message mentions the component.

(From OE-Core rev: e498257ecbec94cec181d73bda57d44335b4dee0)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>