android_os_Power: TARGET_RECOVERY_WRITE_MISC_PART support

Some devices write a padded message to the misc partitions for the
bootloader to pickup and enable recovery mode.  Enable support by
defining the 'misc' mtd partition number using:
TARGET_RECOVERY_WRITE_MISC_PART in BoardConfig.

Requires mtd misc device to be writeable by user system

Change-Id: Iffd88b07696f138d7127acda5b42ede55b468205