wimboot and vdisk_name_len 31 macro

[Mad Butcher]

I like winboot's functionality added by this commit:
https://git.ipxe.org/wimboot.git/commit/...5ae01eeb23

It allow to inject additional files to the loaded WIM, but I want to use long filenames when I add files. I looked at sources and saw in the vdisk.h file 'vdisk_name_len 31' macro.
Is it restriction for virtual disk file system or something else?
Why length set to 31?

I ask this question, because increasing this value to 64 (for example) is don't look like breaking down wimboot functionality.

P.S.
I was a little bit confused when build wimboot from source, because I didn't find any build instructions with description of the build targets, depences etc. These build instructions exist?