ath79: Utilize tplink-safeloader definition

ath79: Utilize tplink-safeloader definition

Currently, tplink-safeloader definition is only used a base for
another common definition.

This patch adjusts tplink-safeloader so it can be actually used
for some targets in generic-tp-link.mk.

This patch is cosmetic except for the order of
"check-size $$$$(IMAGE_SIZE)" and "append-metadata" exchanged
for the tplink_re350k-v1 .

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com> [dealed with
tplink_cpe210-v2 and tplink_cpe210-v3, removed tplink-safeloader-uimage's
extra IMAGE/sysupgrade.bin rule]