board/BuR/kwb: Support modify bootcmd through reset-controller

For some cases it is necessary to modify temporaly the bootcommand.
This can be done by writing into the Scratchregister a specific value:

* 0xCC - modify bootcmd "run netboot"
* 0xCD - modify bootcmd "run netscript"
* 0xCE - modify bootcmd "run mmcboot"

the environment in flash is NOT overwritten.

Signed-off-by: Hannes Petermaier <oe5hpm@oevsv.at>