package/rpi-firmware: update

Updating brings in a new bootloader that enables more free codecs:
    MJPEG, VP6, VP8, Ogg Theora and Ogg Vorbis

Also, the command-line definition has moved to its own file, now.
Although the legacy 'cmdline' variable seems to still be supported
for a little while, better switch now than forget when we upgrade
later.

Update the RPi board readme accordingly.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>