ramips: collect and harmonize TP-Link image variants in common file

This moves the various variants of common device definitions for
TP-Link devices to a common Makefile common-tp-link.mk. This
provides the opportunity to reorganize and move parameters between
individual device definitions and the common ones.

While at it, also use the common definitions for previously
independent definitions where appropriate.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>