Use the specified verity key to sign boot and recovery OTA images.

This ensures that when the verity key is rotated to a release key
both the boot and recovery images will be correctly signed. It does
mean that they will both be signed with the same key for now, but
as that doesn't change the threat model separating them is just a
distant nice-to-have.

Bug: 15725238
Change-Id: I5b75e4346fe0655065643ab553431690cc1a8cb0