Recent refactoring introduced a regression. It ignored second argument
of make_support_list function which was originally true for C2600. The
new generic build_image function always passes false.

This patch allows specifying trailing char in a device specific info. It
also switches Archer C9 to the \0 char to make it compliant with vendor
images.

I verified generated images to be binary identical to the ones that
were created before whole refactoring.

Reported-by: Jo-Philipp Wich <jo@mein.io>
Fixes: fd924d20 ("firmware-utils: tplink-safeloader: use one function for generating images")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>