Add a way to install content not from packages

[keszybz]

This goes against the original idea, but without this development of e.g. systemd features for the initrd will be very awkward. Thus, I think we should allow installing stuff into the initrd from a build directory, or even more than one. So that if you e.g. want to test new glibc and new systemd in the initrd, you can provide two subdirectories and install stuff from there. C.f. systemd/systemd@d12e9bd.

[DaanDeMeyer]

See systemd/mkosi#1229 for my ideas on how to support this. I think doing stuff like this becomes quite natural when mkosi is integrated in a build system like meson.