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 HEAD

Conflicts:
	core/Makefile
	tools/releasetools/ota_from_target_files

Change-Id: Ibbb5470928e91dbc84c060858a37284b9275c5c0