It's not just cheap boards, there aren't that many embedded platforms with mainline support at any price level (and many of the ones that are are community-driv

It's not just cheap boards, there aren't that many embedded platforms with mainline support at any price level (and many of the ones that are are community-driven, not by the OEM). Part of it is a deeply ingrained culture of industrial secrecy, but a lot of it is perceived lack of incentive. Companies perceive little benefit to investing the effort required to submit their patches upstream, and there is also pervasive FUD about hackers examining the code for exploits, other companies stealing their IP etc. It sucks and many people are trying to change it, but progress is very slow. FPGAs are generally in an even worse position.