let's not install empty META files

[hannesm]

for both is_freestanding (earlier is_solo5) and nolibc, there's with current mirage (and mkernel) workflow no need to have META files installed. I tested this with e.g. qubes-mirage-firewall. Would be great if this can be tested and approved by someone else as well.

[hannesm]

I actually think that threads is also misguided, and we may not need that at all. I'll do some experiments.

[hannesm]

See the last commit, still compilation succeeds (of the various unikernels mentioned earlier). I think this was leftovers from mirage3 where ocamlfind was used extensively. These days, all of that can be removed (and thus reducing complexity and wrong information, such as type_of_threads = "posix").

As said, if someone (or more than one) can confirm this, it would be good.

[palainp]

Thank you @hannesm, more is less and less is more!

I tested with success for qubes-mirage-firewall (as you did) and another Xen unikernel. So I approve your changes :D