opkg-build: Remove bashism in sh script

Otherwise this fills logs with output on systems with dash as /bin/sh,
we might as well execute cleanly.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

opkg-build

@@ -239,7 +239,7 @@ rm $tmp_dir/tarX
 
 echo "2.0" > $tmp_dir/debian-binary
 
-if (( $opkext == 1)); then 
+if [ $opkext -eq 1 ]; then 
 	pkg_file=$dest_dir/${pkg}_${version}_${arch}.opk
 else
 	pkg_file=$dest_dir/${pkg}_${version}_${arch}.ipk