package/rpi-firmware: fix config.txt parsing

The rpi-firmware we currently package has a fiscious bug, that happens
in corner cases while reading config.txt (mostly seen on RPi-2, but may
also occur on RPi-1).

Both the DT-aware and DT-unaware branches are affected.

Bump to the latest rpi-firmware to fix that.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>