ath79: get rid of BOARD_NAME for Mikrotik RouterBOARD
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_nand().
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.
By that, BOARD_NAME is obsolete now for this device.
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>