signer security test: special case dogfood channel for app id check

Similar to the canary channel, the dogfood channel images can have their
own app id that is distinct from the board app id.

BUG=chromium-os:25702, chrome-os-partner:8441
TEST=on a dogfood-channel image

Change-Id: Ic993a40d905b224072d325a69e47fdb6633c2e22
Reviewed-on: https://gerrit.chromium.org/gerrit/18039
Tested-by: Gaurav Shah <gauravsh@chromium.org>
Reviewed-by: Scott Zawalski <scottz@chromium.org>
2 files changed