[bug#67534] Requesting review

[Sergey Trofimov]

Hi Maxim,

may I ask your review here, as you've authored qmk firmware 
package. I'm adding `keyboard-source-directory` to the 
`make-qmk-firmware` procedure, so that a custom firmware could be 
built without upstreaming a keyboard definition to the main QMK 
repo. Here is how I use it: 
https://github.com/sarg/keyboard/blob/91b87a830ef6029c1023c0d8fc877c067b12ab1e/guix.scm

Best regards,
Sergey