Add support for update related vendor commands in mn50 by relocating
relevant code from board/cr50 to chip/g.
BUG=b:36910757
BRANCH=None
TEST=./extra/usb_updater/usb_updater -d 18d1:502a build/mn50/ec.bin
Change-Id: Iec0fe5585b5b6eb099f9254dfb0e5b02d5106abc
Reviewed-on: https://chromium-review.googlesource.com/537999
Commit-Ready: Nick Sanders <nsanders@chromium.org>
Tested-by: Nick Sanders <nsanders@chromium.org>
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>