mkfwimage: Add image type definition for WA images

mkfwimage: Add image type definition for WA images

This patch adds a new type of ubiquiti image, the WA image. First seen
on the NanoStation AC loco the generic name implies that we will see
this type of image on more ubiquiti devices thus it makes sense to
implement it in mkfwimage.

The main difference is that WA images are signed. The "END" header has
been replaced by a "ENDS" header followed by a 2048 bit RSA signature.
This signature is not being generated by mkfwimage and filled with 0x00.

Signed-off-by: Tobias Schramm <tobleminer@gmail.com>