usb-boot: Improve messages somewhat explainign that it wants u-boot SPL FEL

usb-boot

 #!/bin/sh -e
 top=`dirname $0`
 if [ $# -lt 2 ]; then
-	echo "Usage: $0 board 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: Board must be specified"; exit 1;)
+board=$1; shift || (echo "ERROR: u-boot-spl.bin must be specified"; exit 1;)
 uboot=$1; shift || (echo "ERROR: u-boot.bin must be specified"; exit 1;)
 bootscr=$top/felboot/ramboot.scr
 if [ ! -f $bootscr ]; then
@@ -32,7 +32,11 @@ case $board in
 	;;
 esac
 if [ ! -f $felboot ]; then
-	echo "ERROR: Can't find fel-boot binary for ${board}"
+	echo "ERROR: Can't find SPL FEL binary ${board}"
+	exit 1
+fi
+if [ `wc -c $felboot | cut '-d ' -f1` -gt 15616 ]; then
+	echo "ERROR: SPL FEL binary too large. Must be the FEL version of SPL"
 	exit 1
 fi
 if [ ! -f $bootscr ]; then