octeon: replace backticks by $(...)

This replaces deprecated backticks by more versatile $(...) syntax.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
[decapitalized patch subject at submitter's request]
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

target/linux/octeon/base-files/lib/upgrade/platform.sh

@@ -85,8 +85,8 @@ platform_check_image() {
 	er | \
 	erlite)
 		local tar_file="$1"
-		local kernel_length=`(tar xf $tar_file sysupgrade-$board/kernel -O | wc -c) 2> /dev/null`
-		local rootfs_length=`(tar xf $tar_file sysupgrade-$board/root -O | wc -c) 2> /dev/null`
+		local kernel_length=$((tar xf $tar_file sysupgrade-$board/kernel -O | wc -c) 2> /dev/null)
+		local rootfs_length=$((tar xf $tar_file sysupgrade-$board/root -O | wc -c) 2> /dev/null)
 		[ "$kernel_length" = 0 -o "$rootfs_length" = 0 ] && {
 			echo "The upgrade image is corrupt."
 			return 1