FreeBSD support

[jd1]

Similar to #504 it would be nice if you could support FreeBSD.
I tried to build clevis, but it fails in luks/tests/meson.build:6:13:

ERROR: Program 'cryptsetup' not found or not executable

I don't need support for luks, I just want to use clevis decrypt with a tang server in a script.
It would be nice if I could compile clevis with support for and clevis encrypt tang and clevis decrypt.