White screen for 10.12.2 upgrade on mac mini
I experienced the same white screen issue after updating to 10.12.2 and was forced to restore from a previous time machine backup (after trying to repair the drive, resetting the PRAM/NVRAM and booting safe mode failed to work).  Stupidly, I tried updating to 10.12.2 again on the restored version and during the reboot process it went back to the white screen.

Don't install 10.12.2 if you have this issue.