scripts/mkusers: allow the creation of groups alone

Let mkusers create groups alone, useful for supplementary permissions in
udev/systemd for example where users can be added to later at runtime.
Use a magic string "-" to signal that user creation should be skipped.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>