keys: avoid duplicating recovery keys

Setting PRODUCT_EXTRA_RECOVERY_KEYS duplicates the default key and
makes it so we can't set an extra key earlier in the build

Change-Id: I239d9994d9ad40b1779877ea2e89b55bd02e4c00
ota: Allow devices to skip generation of recovery-from-boot.p

* Not all devices have a real recovery partition, so those devices
  should skip this generation step.

Change-Id: I4abc3f92b879e6997e4874543ec9772c3179b1d0
Merge branch 'cm-11.0' of https://github.com/Cyanogenmod/android_build into cm-11.0

Conflicts:
	tools/releasetools/ota_from_target_files

Change-Id: I62b3e2827b2c4744acbb090c259a6b139cb1c405