fastboot: add new oem command for post wipe userdata

When Android userdata partition has been erased in fastbootd, call
oem specific API doOemSpecificErase() to wipe other userdata in
device.

If oem doesn't implement this specific API in fastboot_hal lib,
fastbootd will receive 'NOT_SUPPORTED' return status.

Bug: 169173873
Change-Id: I9b6a5a4aaed31d1168e633418b189f9bb6d34d01