scripts/gen_image_generic.sh: replace -o with if/&&

-o is not well defined.

Found with shellcheck.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

scripts/gen_image_generic.sh

@@ -1,10 +1,10 @@
 #!/usr/bin/env bash
 # Copyright (C) 2006-2012 OpenWrt.org
 set -e -x
-[ $# == 5 -o $# == 6 ] || {
+if [ $# -ne 5 ] && [ $# -ne 6 ]; then
     echo "SYNTAX: $0 <file> <kernel size> <kernel directory> <rootfs size> <rootfs image> [<align>]"
     exit 1
-}
+fi
 
 OUTPUT="$1"
 KERNELSIZE="$2"