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>
diff --git a/include/configs/bur_am335x_common.h b/include/configs/bur_am335x_common.h
index c74140b..b441fa0 100644
--- a/include/configs/bur_am335x_common.h
+++ b/include/configs/bur_am335x_common.h
@@ -13,6 +13,7 @@
 #define __BUR_AM335X_COMMON_H__
 /* ------------------------------------------------------------------------- */
 #define BUR_COMMON_ENV \
+"usbscript=usb start && fatload usb 0 0x80000000 usbscript.img && source\0" \
 "defaultip=192.168.60.253\0" \
 "defaultsip=192.168.60.254\0" \
 "netconsole=echo switching to network console ...; " \