brcm2708: restore /boot/config.txt before reboot

brcm2708: restore /boot/config.txt before reboot

The Raspberry Pi bootloader reads configuration values from config.txt
in the boot partition. This file allows to specify the amount of memory
to assign to the GPU, the license keys for hardware MPEG-2 and VC-1
decoding, Device Tree parameters and overlays, and lots of other things.

Since sysupgrade only restores the configuration after booting the newly
flashed image, these values will not be active, even if sysupgrade would
save /boot/config.txt. To solve this, add the file to the files to be
backed up, and restore it in platform_copy_config, before reboot.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>