Compile to OpenBSD and NetBSD

[Jacalz]

Is your feature request related to a problem? Please describe:

Fyne 2.2.0 is adding support for OpenBSD and NetBSD. I don't know how many users we will get for these platforms (the usage of FreeBSD is a lot lower than most other platforms) but it might make sense if there are enough users that show an interest in it.

Is it possible to construct a solution with the existing API?

As long as it is possible to cross-compile OpenBSD and NetBSD, then yes, it should be possible.

Describe the solution you'd like to see:

Add fyne-cross openbsd and fyne-cross netbsd commands.