mvebu: fix sysupgrade failure

Linksys devices do no pass $cmd at all.

Fixes: #16148
Fixes: 715634e6d1 ("base-files: upgrade: nand: use "cmd" argument for extracting command")
Signed-off-by: Boris Krasnovskiy <borkra@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/16690
Signed-off-by: Robert Marko <robimarko@gmail.com>

package/base-files/files/lib/upgrade/nand.sh

@@ -300,7 +300,7 @@ nand_upgrade_fit() {
 # Write images in the TAR file to MTD partitions and/or UBI volumes as required
 nand_upgrade_tar() {
 	local tar_file="$1"
-	local cmd="$2"
+	local cmd="${2:-cat}"
 	local jffs2_markers="${CI_JFFS2_CLEAN_MARKERS:-0}"
 
 	# WARNING: This fails if tar contains more than one 'sysupgrade-*' directory.