move key loading to verifier code

Add an option to verifier_test to load keys from a file, the way the
recovery does.

Conflicts:
	verifier_test.c

Change-Id: Icba0e391164f2c1a9fefeab4b0bcb878e91d17b4
recovery: save logs from the last few invocations of recovery

Extends the last_log mechanism to save logs from the last six
invocations of recovery, so that we're more likely to have useful logs
even if the device has repeatedly booted into recovery.

Conflicts:
	recovery.c

Change-Id: I08ae7a09553ada45f9e0733fe1e55e5a22efd9f9