Separate boot and recovery image signing from dm-verity

Allow dm-verity to be enabled without boot and recovery images being
signed. This makes it possible to enable only dm-verity to detect
corruption without confusing bootloaders that do not understand signed
images.

Bug: 19985143
Change-Id: Ie52c6ff595faa7a5c1f1bc1b37f6899c4d0c7001