package/systemd: add setting for systemd default.target

This adds the option to set systemd's default.target in
the System Configuration subheading if systemd is
specified as the init system.

The argument for default.target is specified pre-build
as opposed to overriding the hardcoded "multi-user.target"
symlink with post-build scripts or a rootfs overlay

Signed-off-by: Sen Hastings <sen@phobosdpl.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/systemd/systemd.mk

@@ -571,7 +571,7 @@ SYSTEMD_CONF_OPTS += -Dfallback-hostname=$(SYSTEMD_FALLBACK_HOSTNAME)
 endif
 
 define SYSTEMD_INSTALL_INIT_HOOK
-	ln -fs multi-user.target \
+	ln -fs "$(call qstrip,$(BR2_PACKAGE_SYSTEMD_DEFAULT_TARGET))" \
 		$(TARGET_DIR)/usr/lib/systemd/system/default.target
 endef
 

system/Config.in

@@ -154,6 +154,17 @@ source "$BR2_BASE_DIR/.br2-external.in.init"
 
 endchoice
 
+config BR2_PACKAGE_SYSTEMD_DEFAULT_TARGET
+	string "The default unit systemd starts at bootup"
+	default "multi-user.target"
+	depends on BR2_INIT_SYSTEMD
+	help
+	  Specify the name of the unit configuration file to be started
+	  at bootup by systemd. Should end in ".target".
+	  ex: multi-user.target
+
+	  https://www.freedesktop.org/software/systemd/man/systemd.special.html#default.target
+
 choice
 	prompt "/dev management" if !BR2_INIT_SYSTEMD
 	default BR2_ROOTFS_DEVICE_CREATION_DYNAMIC_DEVTMPFS