package/systemd: bump to version 252.4

Following files changed *license* from CC0-1.0 to MIT-0:

-   src/systemctl/systemd-sysv-install.SKELETON
-   config files
-   examples under /network

The file LICENSES/MIT-0.txt has been added for this reason,
hashes added, corrected for LICENSES/README.md.

Kernel version 3.15 is now the bare minimum, but only
version 4.15 and higher are fully supported and tested:

-   bump kernel header dependencies
-   correct kconfig description
-   improvements to make the kconfig description better reflect
    the information from the README

Set new options to their default, remove -Defi-cc option.

Set -Ddbus=false to ensure dbus dependency is runtime only.

Set -Ddbus-interfaces-dir=no as interface XML file generation is
not supported when cross compiling.

Set -Ddefault-user-shell=/bin/sh to the always available shell.

Signed-off-by: Norbert Lange <nolange79@gmail.com>
Reviewed-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>