fastboot: add a new oem command for post wipe userdata

When user input 'fastboot erase userdata' in fastbootd, may need an
oem specific API doOemSpecificErase() to wipe other userdata in device.
If oem doesn't need this specific API, oem could return NOT_SUPPORTED
to fastbootd.

Bug: 169173873
Change-Id: Ie12ede31ef071a3c15265777b55746536a861292
6 files changed