ath79: fix Mikrotik wAP G-5HacT2HnD mtd partitions

ath79: fix Mikrotik wAP G-5HacT2HnD mtd partitions

In RouterBOARD parlance there never was an "art" partition.
This partition has always been named 'hard_config' on ar71xx.

This partition contains more than just ART (Atheros Radio Test) data. It
includes the hardware description (product code, serial, board
identifier, name, hardware options, MAC address), as well as other bits
affecting the operation of RouterBoot.
To avoid confusion with regular ART data, this partition is renamed in
line with historical ar71xx and ramips nomenclature as 'hard_config'.

This commit fixes the previous support files and implements the nested
RouterBoot partition scheme as already used by ramips-based SPI-NOR
RouterBOARD DTSes, as previously reviewed and implemented in
bbe2cf657ca7 ("ramips: fix RBM11G partitioning").

Tested-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>
Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>
[minor commit title/message adjustments]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>