docker-engine: fix journald logging driver

The Docker engine can optionally log to systemd-journald. For this
driver to work correctly, Docker needs to build against
systemd-journald's client library.

This patch conditionally adds a build-time dependency on systemd and
enables compiling the journald driver in docker-engine if systemd is
used as the Buildroot init process.

Signed-off-by: Christian Stewart <christian@paral.in>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/docker-engine/docker-engine.mk

@@ -42,6 +42,11 @@ DOCKER_ENGINE_BUILD_TAGS += seccomp
 DOCKER_ENGINE_DEPENDENCIES += libseccomp
 endif
 
+ifeq ($(BR2_INIT_SYSTEMD),y)
+DOCKER_ENGINE_BUILD_TAGS += journald
+DOCKER_ENGINE_DEPENDENCIES += systemd
+endif
+
 ifeq ($(BR2_PACKAGE_DOCKER_ENGINE_DAEMON),y)
 DOCKER_ENGINE_BUILD_TAGS += daemon
 DOCKER_ENGINE_BUILD_TARGETS += dockerd