b43-firmware: new package

This package allows to download the Broadcom Wifi drivers, extract the
firmware from them, and install them in /lib/firmware, so that they
can be used by the open-source kernel driver b43.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>