pushimage: add support for signing accessories images

'accessory_usbpd' and 'accessory_rwsig' are used to sign standalone
accesory MCU firmwares (right now, the former are all the USB type-C
accessories: e.g. Zinger power supply, DingDong and HoHo display
dongles, the latter is the Smaug wireless keyboard).

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>

BUG=chrome-os-partner:46635
TEST=bin/pushimage --debug --test-sign premp --sign-types accessory_rwsig --board smaug gs://chromeos-image-archive/smaug-firmware/R45-7132.239.0
TEST=./scripts/pushimage_unittest

Change-Id: I6481c68061f50b9acc3656a1167d64ced79ec011
Reviewed-on: https://chromium-review.googlesource.com/308725
Commit-Ready: Vincent Palatin <vpalatin@chromium.org>
Tested-by: Vincent Palatin <vpalatin@chromium.org>
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>
3 files changed