Fix usb-boot usage message.

When - is specified usb-boot interprets it as kernel so the usage
message is clearly bogus.
Signed-off-by: Michal Suchanek <hramrach@gmail.com>
Signed-off-by: Henrik Nordstrom <henrik@henriknordstrom.net>

usb-boot

 #!/bin/sh -e
 top=`dirname $0`
 if [ $# -lt 2 ]; then
-	echo "Usage: $0 u-boot-spl.bin u-boot.bin [boot.scr|-] [kernel script.bin [initramfs]]"
+	echo "Usage: $0 u-boot-spl.bin u-boot.bin [boot.scr] [kernel script.bin [initramfs]]"
 	exit 1
 fi
 board=$1; shift || (echo "ERROR: u-boot-spl.bin must be specified"; exit 1;)