ath79: fix build of big images for TP RE355v1, RE450v1 & v2

ath79: fix build of big images for TP RE355v1, RE450v1 & v2

After merge of https://github.com/openwrt/openwrt/pull/20709 I noticed,
that https://mirror-03.infra.openwrt.org/snapshots/targets/ath79/tiny/
still does not contain the build for RE450.

I analyzed the problem and noticed, that only builds which fits into
original size can be build and if the image is larger, it fails with
the following error: "WARNING: Image file .../openwrt/build_dir/
target-mips_24kc_musl/linux-ath79_tiny/tmp/
openwrt-ath79-tiny-tplink_re450-v1-squashfs-sysupgrade.bin is too big:
  > 7864320".

This error is quite misleading as the image is smaller than 7864320. The
reason for this error is that default tplink-safeloader has hardcoded
factory partition structure and fails to generate any file.

Rather then fixing tplink-safeloader I followed approach from commit
ebd5e5fb5359 ("ramips: switched TP-Link RE305 v1 to new partition layout")
and switched away from "tplink-safeloader sysupgrade".

I did not include "IMAGES := sysupgrade.bin", because with tiny target it
is still possible to locally build even factory image.

Fixes commit e768731dc8e9 ("ath79: switch TP-Link RE355 v1, RE450 v1 and
RE450 v2 to mtd-concat").

Signed-off-by: Radek Dostál <rd@radekdostal.com>
Link: https://github.com/openwrt/openwrt/pull/21158
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>