[Systemd] Change the default target

[rambip]

While installing my bluebuild image, I was confused for a long time because gdm would not start. After debugging for hours, I noticed that gdm did not start because the default systemd target was multi-user.target and not graphical.target.
I don't really know how systemd works, so I tried to add systemctl set-default graphical.target using the snippet feature. It did not work, but I'm not surprised too much since this feature is enabled as a systemd symlink and not as a file.
Is there a workaround ? If not, it would be nice to be able to specify the default target, and have it documented on the site.