Medkit: target model has to be provided

I was fiddling around medkits for a while as somebody on IRC complained about documentation and build process. Documentation will be handled in !326 (merged).

I wanted to add a stage in Gitlab CI to know, but I am not able to generate medkit in branch HBL.

HBL pipeline: https://gitlab.nic.cz/turris/turris-build/-/jobs/482658 Output: DIE:Target model has to be provided by BOARD environment variable. I tried to generate medkits with this command: ./generate_medkits -t omnia and I tried to use even script, which is located in helpers folder and still no go for HBL branch. I can reproduce this even locally.

When I took a script from HBK and move it to HBL, it seems like it was doing something: https://gitlab.nic.cz/turris/turris-build/-/jobs/482694 but it didn't say it was successful or not. I can build HBK medkits just fine even locally.