brcm2708: implement sysupgrade image check

Implement a crude but functioning sysupgrade image check for the
Raspberry Pi. The code only checks if the master boot record boot
signature (0x55aa) is present in the first 512-bytes at the correct
location. This can prevent the odd bricking of a system when flashing
the wrong file.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>