faft: Re-use VerifyImageAndGetId for ensuring a test image in USB

Reduce the code for checking USB disk containing a test image and re-use
VerifyImageAndGetId which can do the same thing.

BUG=chromium-os:34395
TEST=manual, run the following test without using Beaglebone:
$ run_remote_tests.sh --board daisy --remote dut RecoveryButton/control$

Change-Id: Ie4215c051cd663e6632a8eb9306d781208ff3522
Reviewed-on: https://gerrit.chromium.org/gerrit/36051
Commit-Ready: Tom Wai-Hong Tam <waihong@chromium.org>
Reviewed-by: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Tom Wai-Hong Tam <waihong@chromium.org>
1 file changed