recovery: datamedia support

Change-Id: I4cef82973a15111bee92cd7c81f0e1db8d211991

recovery: Fix recursive rm wipe of data

Android 6 re-introduced MCS/MLS SELinux contexts.  Unforunately,
this broke our previous SELinux model for walking /data and unlinking
everything but media.  Fix this by mounting /data with a temporary
SELinux context that works with the previous model.

Ticket: CYNGNOS-1747
Change-Id: Id87ad3bb357102c3a8bd7c1417183d788ef858a0