systemd: Fail if kernel features are missing.

This has rendered my system unbootable, because I forgot to enable AUTOFS4 in my
custom kernel. In addition to AUTOFS4, this includes (hopefully) all other
kernel features needed by systemd, as listed in the README:

REQUIREMENTS:
  Linux kernel >= 2.6.39
    with devtmpfs
    with cgroups (but it's OK to disable all controllers)
    optional but strongly recommended: autofs4, ipv6

Autofs4 is not a requirement here, but in our case it turns out that the system
is not able to boot properly with a LUKS-enabled system (or at least not on _my_
system).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>