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.

* psyke83: updated for cm-12.0 branch

Conflicts:
	core/Makefile
	tools/releasetools/ota_from_target_files

Change-Id: I4abc3f92b879e6997e4874543ec9772c3179b1d0