Only disable recovery flash if it is truly "stock" recovery

 Some roms need the recovery-from-boot.p for auto update
 of recovery. Typically the use install-[romname]-recovery.sh.
 So only disable if install-recovery.sh is present.

Change-Id: I01f89afc87b9d375d67b44b8b5e9228868a47d69
Merge remote branch 'omni/android-4.4' into android-4.4