package/docker-engine: carry the tini init symlink

tini can be used standalone, not just as an init injected by docker.
As such, creating the docker-engine's init symlink does not belong to
tini itself, but to docker-engine.

This will also help later, when we allow docker-engine to use another
init alternative, catatonit (already paclkaged in Buildroot, but not
integrated with docker).

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Christian Stewart <christian@aperture.us>
Cc: Fiona Klute (WIWA) <fiona.klute@gmx.de>
Tested-by: Fiona Klute (WIWA) <fiona.klute@gmx.de>
Signed-off-by: Julien Olivain <ju.o@free.fr>

package/docker-engine/docker-engine.mk

@@ -35,6 +35,14 @@ DOCKER_ENGINE_DEPENDENCIES += systemd
 DOCKER_ENGINE_TAGS += systemd journald
 endif
 
+ifeq ($(BR2_PACKAGE_DOCKER_ENGINE_DOCKER_INIT),y)
+define DOCKER_ENGINE_INIT
+	mkdir -p $(TARGET_DIR)/usr/libexec/docker
+	ln -sf ../../bin/tini $(TARGET_DIR)/usr/libexec/docker/docker-init
+endef
+DOCKER_ENGINE_POST_INSTALL_TARGET_HOOKS += DOCKER_ENGINE_INIT
+endif
+
 ifneq ($(BR2_PACKAGE_DOCKER_ENGINE_DRIVER_BTRFS),y)
 DOCKER_ENGINE_TAGS += exclude_graphdriver_btrfs
 endif

package/tini/tini.mk

@@ -31,8 +31,6 @@ endef
 
 define TINI_INSTALL_TARGET_CMDS
 	$(INSTALL) -D -m 0755 $(@D)/bin/tini $(TARGET_DIR)/usr/bin/tini
-	mkdir -p $(TARGET_DIR)/usr/libexec/docker
-	ln -sf ../../bin/tini $(TARGET_DIR)/usr/libexec/docker/docker-init
 endef
 
 # Tini's CMakeLists.txt is not suitable for Buildroot.