assert recovery finger/thumbprints, not system

Block OTAs don't have the system partition mounted and so can't use
file_getprop on the system partition.  Make assertions look at the
recovery system finger/thumbprints, which should be the same as the
ones on the system partition (even for sprout devices).

Change-Id: Ie5d329d13beab4b428e37f75da9b9e1b8ceb35bc