There are already two targets (lantiq, ramips) which use mktplinkfw2
tool for creating images. This de-duplicates code, introduces two new
build commands: tplink-v2-header, tplink-v2-image and makes use of
them in place of old, (sub)target specific ones.
Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>
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