fastboot: add new oem command for post wipe userdata

When user input fastboot erase userdata, need a follow
up oem command to wipe other user data in device.
We support this new postwipedata command in
"fastboot erase userdata" only.

Bug: 150929955
Change-Id: I9b6a5a4aaed31d1168e633418b189f9bb6d34d01
Ignore-AOSP-First: I9b6a5a4aaed31d1168e633418b189f9bb6d34d01