New rules: everything under "config" that isn't actually configuration
(e.g. build products) will in future live in config.system. This is
the first step.
systemconfig (a.k.a "activate") is run from the initramfs. Converting
it from a shell script to an executable means it doesn't depend on
there being a shell in the initramfs
the jffs2 filesystem contains only /nix/store and a script which is
run in early init (initramfs) and is responsible for recreating
"traditional" directories (/bin /etc/**/* /var &c) based on the
configuration.
this is tested only in qemu so far and could use some cleanup
