mkosi: Introduce particle profile

mkosi: Introduce particle profile

Unfortunately the current mkosi partitioning setup is a bit too
avant-garde for the integration tests. Both in that distributions
aren't ready for it yet (some more than others), and that software
which we depend on in the integration tests isn't ready for it yet
(e.g. libselinux does not read its configuration from /usr).

Let's switch back to a more boring partioning setup by default but
keep the fancy stuff around as a mkosi profile. This means that it
can still be used for manually testing stuff by running
"mkosi --profile particle -f qemu".