combo-layer: add sync-revs command

When starting to use combo-layer, or if someone else is using it too,
the local last_revision may be incorrect.

This command will forcibly update the last_revision config values to the
latest SHA on the remote branch that is tracked.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>