uboot-imx

board/BuR/common: rename "usbupdate" environment to "usbscript"

usbupdate in real does allways load some script from usb-storage and execute
it, on all B&R targets.

So we do following 2 things:
- rename it to what it really does
- move it from boards to common environment

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