package/systemd: hybrid cgroupfs hierarchy for docker compatibility

Docker fails to start with "Devices cgroup isn't mounted" as of systemd 243.
According to the systemd documentation:

  systemd now defaults to the "unified" cgroup hierarchy setup during
  build-time, i.e. -Ddefault-hierarchy=unified is now the build-time default.
  Previously, -Ddefault-hierarchy=hybrid was the default. [...] Downstream
  production distributions might want to continue to use
  -Ddefault-hierarchy=hybrid (or even =legacy) for their builds as unfortunately
  the popular container managers have not caught up with the kernel API changes.

Changing this option to "hybrid" or "legacy" fixes the Docker startup.

Reference: opencontainers/runc#654

Signed-off-by: Christian Stewart <christian@paral.in>
Tested-by: Jérémy Rosen <jeremy.rosen@smile.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/systemd/systemd.mk

@@ -26,6 +26,7 @@ SYSTEMD_CONF_OPTS += \
 	-Dima=false \
 	-Dldconfig=false \
 	-Ddefault-dnssec=no \
+	-Ddefault-hierarchy=hybrid \
 	-Dtests=false \
 	-Dsplit-bin=true \
 	-Dsplit-usr=false \