ath79: switch TP-Link RE355 v1, RE450 v1 and RE450 v2 to mtd-concat

ath79: switch TP-Link RE355 v1, RE450 v1 and RE450 v2 to mtd-concat

This patch utilizes unused and empty flash space in the original partition
layout and increase maximum image size by 1664 KiB.

Tested on RE450v1 by running sysupgrade with build of size 5697 KiB and
checking that df shows 1988 1K-blocks for "/overlay".

Inspired by commit ebd5e5fb5359 ("ramips: switched TP-Link RE305 v1 to new
partition layout").

Signed-off-by: Radek Dostál <rd@radekdostal.com>
Link: https://github.com/openwrt/openwrt/pull/20709
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>