Hi, I would like to compile the firmware myself, but cannot find resource how to do so, or if they are very old and seem not to work anymore.
Is it still supported that users can work themselves on the firmware?
I think this dood is under a 5 eyes NSL or something… doesn’t say much. 
It was never supported for users to develop their own firmware for OnlyKey. OnlyKey firmware is open source so that source could be used on other open source projects.
but is it technically possible to compile the source thats available online or are there some important things that are missing which would make it impossible for a user to compile and deploy the firmware?
Yes, there have been users who compiled firmware themselves using Arduino 1.6.5. There is just no official support for that. Production OnlyKeys will only load firmware signed by CryptoTrust.
The latter one means its impossible to do it, because there is no way to get the firmware signed, so one would need a development key, correct?