ar71xx: remove hard-coded folder name from Mikrotik RB upgrade

So far, specifying "BOARD_NAME := routerboard" is required by the
upgrade code of Mikrotik NAND devices, as "sysupgrade-routerboard"
is hardcoded in platform_do_upgrade_mikrotik_rb().

This patch replaces the latter with a grep for the name like it
is already done in nand_upgrade_tar() in /lib/upgrade/nand.sh.

This should enable upgrades from ar71xx to ath79 without setting
BOARD_NAME for the latter.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(backported from commit 281785d74fcc70153c0741be36ee1f9d05e74f6f)

target/linux/ar71xx/base-files/lib/upgrade/platform.sh

@@ -773,8 +773,13 @@ platform_nand_pre_upgrade() {
 		local fw_mtd=$(find_mtd_part kernel)
 		fw_mtd="${fw_mtd/block/}"
 		[ -n "$fw_mtd" ] || return
+
+		local board_dir=$(tar tf "$1" | grep -m 1 '^sysupgrade-.*/$')
+		board_dir=${board_dir%/}
+		[ -n "$board_dir" ] || return
+
 		mtd erase kernel
-		tar xf "$1" sysupgrade-routerboard/kernel -O | nandwrite -o "$fw_mtd" -
+		tar xf "$1" ${board_dir}/kernel -O | nandwrite -o "$fw_mtd" -
 		;;
 	wi2a-ac200i)
 		case "$(fw_printenv -n dualPartition)" in