buildroot

package/systemd: Create user only when needed

We don't want to create a bunch of group and users that might not be
used, it's more elegant to create them when the associated features is
selected.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>