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
Revert "Revert "releasetools: bring back boot.img handling""

This reverts commit 9f7b879d76d3f6f405b8754b2aeee823ba7e61fa.