systemd-boot: Make sure /etc/machine-id exists

This leads to the following error when trying to install a new machine
where the machine ID wasn't yet initialized during boot:

Failed to get machine did: No such file or directory

In addition this was also detected by the simpleUefiGummiboot installer
test.

So let's generate a fallback machine ID by using
systemd-machine-id-setup before actually running bootctl.

Tested this by running the installer.simpleUefiGummiboot test, it still
fails but not because of the machine ID.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @edolstra, @shlevy, @mic92
Fixes: #22561