recovery: Add support for performance control

 * Recovery will now set "recovery.perf.mode" to 1 before a
   backup/restore operation begins, and will set it back to 0
   when the operation is complete. It will also do this during
   package installations.
 * This allows devices to boot into recovery in a low-power mode,
   and bring up more cores and adjust other settings. Pigz is
   multithreaded, and enabling all cores vastly improves performance of
   compressed backups.
 * Device maintainers who wish to use this should configure their
   init.recovery script to trigger off of this property.

Change-Id: I174e45215ad1cd34a34b1af4486a64fb6e54134b