package/systemd: bump to 243-78

package/systemd: bump to 243-78

The systemd project maintains a separate repository on github where patches
are backported on top of released version by the systemd maintainers.

This patch changes the SYSTEMD_SITE to point on that repository and points
to the latest version of v243, which was the previous version used by
systemd.

Unfortunately, upstream does not tag any version,so we use 'git describe'
as a SYSTEMD_VERSION

Signed-off-by: Jérémy Rosen <jeremy.rosen@smile.fr>
Reviewed-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>