rpi-firmware: New package

Introducing a package to install pre-built binaries for the bootloader and
the GPU firmware for the RaspberryPi board.

[Peter: rename to rpi-firmware, add link to http://elinux.org/RPiconfig

]
Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>