ar71xx: base-files: fix board detect on new MikroTik devices

ar71xx: base-files: fix board detect on new MikroTik devices

Move all MikroTik devices to new function to increase script execution
speed.

Machine name in new version of MikroTik RouterBOARD devices add "RB"
before model name:

 Old machine name: MikroTik RouterBOARD 951Ui-2nD
 New:              MikroTik RouterBOARD RB951Ui-2nD

So this patch should fix it for all currently supported MikroTik boards.

Signed-off-by: Henryk Heisig <hyniu@o2.pl>
[rebased,commit message facelift,script fixes]
Signed-off-by: Petr Štetiar <ynezz@true.cz>
[spotted missing 922UAGS-5HPacD]
Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
(cherry picked from commit acf2b6c8881b432530bd98fa86753bf6a3546ff7)
[backport: do not add boards not supported in 18.06]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>