Fix tests that rely on rootdev specifying a real block device.

Turning on DM-Verity a week ago changed rootdev output and broke these tests.

BUG=partner
TEST=manual: run each test by hand.

The factory_ExternalStorage change appears unused, but I changed it here to
better reflect current usage.

Change-Id: I06e0215fabaabbdd603d63f8e47a0b0a0c3c2ec0

Review URL: http://codereview.chromium.org/3256005
3 files changed