ar71xx: mikrotik: fix upgrading beyond 19.07 using sysupgrade

ar71xx: mikrotik: fix upgrading beyond 19.07 using sysupgrade

Newer releases uses a different path within sysupgrade to store files.

This leads to sysupgrade currently being broken when trying to upgrade
MikroTik devices to this newer model, even when using the Force flag.

Fix this by trying the newer location for verification if the standard
one fails.

This patch allows to easily use sysupgrade for upgrading Mikrotik
devices beyond 19.07 releases.

Using RB922 as an example:

boardname --> rb-922uags-5hpacd

sysupgrade archive main folder name:

Within 19.07 -->  sysupgrade-mikrotik/
Beyond 19.07 -->  sysupgrade-mikrotik_routerboard-922uags-5hpacd/

Signed-off-by: Koen Vandeputte <koen.vandeputte@citymesh.com>