systemd: do not create machine-id

systemd: do not create machine-id

There is no reason to have an emtpy machine-id as part of the systemd
package.  Either:

i)  the filesystem is writable and the file will be created
automatically; or
ii) the filesystem is read-only, in which case the empty machine-id file
should be created as part of the read-only-rootfs tweaks.

Signed-off-by: Jonas Bonn <jonas@norrbonn.se>
Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>