To increase the security of wiping A/B devices, let uncrypt write
wipe package in misc partition. Then recovery verifies the wipe
package before wiping the device.
Bug: 29159185
Change-Id: I186691bab1928d3dc036bc5542abd64a81bc2168
init and vold also need to write bootloader message, so
split this function from uncrypt into a separate library.
Bug: 27176738
Change-Id: If9b0887b4f6ffab6162d9cb47a6ceb7eedd60b4d