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>

board/BuR/kwb/board.c

@@ -214,8 +214,8 @@ int board_late_init(void)
 			    gpio_get_value(PUSH_KEY) && 1 == cnt) {
 				lcd_position_cursor(1, 8);
 				lcd_puts(
-				"updating U-BOOT from USB ...           ");
-				setenv("bootcmd", "run usbupdate");
+				"starting u-boot script from USB ...    ");
+				setenv("bootcmd", "run usbscript");
 				cnt = 4;
 				break;
 			} else if ((!gpio_get_value(ESC_KEY) &&