usb-boot: fix small typo

Signed-off-by: Alejandro Mery <amery@geeks.cl>

usb-boot

@@ -6,7 +6,7 @@ if [ $# -lt 2 ]; then
 fi
 board=$1; shift || (echo "ERROR: Board must be specified"; exit 1;)
 uboot=$1; shift || (echo "ERROR: u-boot.bin must be specified"; exit 1;)
-bootscr=$tob/felboot/ramboot.scr
+bootscr=$top/felboot/ramboot.scr
 if [ ! -f $bootscr ]; then
 	bootscr=$top/bin/ramboot.scr
 fi