buildroot

package/systemd: use 'menuconfig'

systemd has a bunch of options now, so use 'menuconfig' to the group the
options in a submenu.

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