mvebu: gen_mvebu_sdcard_img.sh: replace let with $(())

let is a bashism.

Found with shellcheck.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
[add prefix to commit title]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

target/linux/mvebu/image/gen_mvebu_sdcard_img.sh

@@ -79,6 +79,6 @@ while [ "$#" -ge 2 ]; do
 	) | dd of="$OUTFILE" bs=512 seek=$(($1 / 512)) conv=notrunc 2>/dev/null
 	printf "Done\n"
 
-	let i=i+1
+	i=$((i+1))
 	shift; shift
 done