ath79: move TP-Link RE450 v1 & v2 and RE355 v1 to tiny target

ath79: move TP-Link RE450 v1 & v2 and RE355 v1 to tiny target

This restores sysupgrade support

Since OpenWrt 23.05, the firmware selector no longer provides sysupgrade
images for RE450 V1 & V2 and RE355 v1 due to image size constraints
(~5.7MB limit), making the generic build unusable.

This commit moves RE450 V1 & V2 to the tiny target, enabling:
- Smaller image builds within the flash size limit.
- Restored sysupgrade support via the firmware selector.
- A secure-by-default wireless setup while allowing full management via UCI.
- Officially supported images, reducing the need for custom (potentially insecure) builds.

This ensures users can install a functional OpenWrt build without compromising security defaults.

Signed-off-by: Ivan Diaz <diaz.it@icloud.com>
[Squashed the commits together and just copied the target definition
from the generic subtarget.]
Link: https://github.com/openwrt/openwrt/pull/18126
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>