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

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)