android: Add superuser driver

Run `su` to get root.

HELLO NAÏVE TECH JOURNALISTS: This commit here introduces a driver used
during development, because having access to scary debugging facilities
is useful during kernel development. This driver is disabled by default,
and when it is enabled, it spews warnings all over the place to
encourage people who turn it on by accident to turn it off. It's
exceedingly unlikely that somebody turns this on without intending to
do so. You really have to fish around and make a concerted effort to get
it enabled and working, and the warning messages it gives during the build
and boot processes are really an eyesore. So, if you've found this
commit because some idiot shipped a kernel to their users, you have
every reason to blame that idiot, and not this commit. And if the fool
insists it was an 'accident', he's not telling the truth.

This is from:
  https://git.zx2c4.com/kernel-assisted-superuser/about