package/rpi-firmware: only install one firmware file

Since only one firmware is used to boot the Raspberry Pi, there is no
reason to install all of them.

Add an option to select what firmware to install.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>