mcf: Add --clean option for non-interactive updates on jenkins

:Release Notes:
On jenkins builds it's useful to be able to clean all local changes
and always use clean layer defined in weboslayer.py.

:Detailed Notes:
This also allows to updating to HEAD of rebased repo.

:Testing Performed:
Only build tested.

:QA Notes:
No change to image.

:Issues Addressed:
[GF-54390] mcf: Support updating HEAD from rebased branches

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>
Change-Id: I1772f13b3b7d6a3204eff44a6970e3efdfa28ef9
Reviewed-on: https://g2g.palm.com/4768
Reviewed-by: DCO Verification
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Tested-by: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>