rpi-userland: new package

Introducing a package to build the userland part of the Raspberry,
needed by anyone who would want to build a rootfs for a RaspberryPi.

[Peter: fixup Config.in (rename, move, arm dep, comment, white space)]
Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>