package/systemd: explicitly configure split-usr=false and split-bin=true

meson-based build of systemd runs a couple of checks on the structure
of the *host* system which will incorrectly configure systemd if build
host configuration doesn't match Buildroot target.

One can also find the following in the NEWS file:

* A new -Dsplit-bin= build configuration switch may be used to specify
  whether bin and sbin directories are merged, or if they should be
  included separately in $PATH and various listings of executable
  directories. The build configuration scripts will try to autodetect
  the proper values of -Dsplit-usr= and -Dsplit-bin= based on build
  system, but distributions are encouraged to configure this
  explicitly.

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>