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
(cherry picked from commit 5c84893be3a81be5f67af3cef3190798c61cc1c5)

Conflicts:
	core/Makefile
	tools/releasetools/ota_from_target_files