Rework tplink-v2-header and tplink-v2-image Build define to ignore error
if mktplinkfw2 errors out.
This is to handle situation when the image is too big and can't be
generated or prev check-image calls deleted the source file as it's too
big.
This aligns to the pattern used by tplink-v1-image.
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>