systemd: don't initialize machine-id, do initialize ld.so.conf

systemd for a while now has properly initialized machine-id if the file
is missing or blank. Switch to letting it do that by removing or bogus
machine id if it is present.

systemd does not always set up ld.so.conf early enough, switch to doing
so in the initrd. This removes the need for one of our systemd patches.
Merge pull request #39 from marineam/systemd2

systemd: don't initialize machine-id, do initialize ld.so.conf
diskless-generator: check value of mount.usr= and usr=

systemd 217 added mount.usr= which replaces our own usr= option.
Avoid engaging the diskless logic if the new option is used.