package/systemd: allow empty systemd default target

Currently, we forcefully require that a default target be specified.
However, systemd does install a default target, but it's not
possible to just use that target.

Allow the user to blank-out the config setting to not override
whatever systemd installed and keep that.

Signed-off-by: Norbert Lange <nolange79@gmail.com>
[yann.morin.1998@free.fr:
  - introduce the usual qstripped inter;ediate variable
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/systemd/systemd.mk

@@ -570,17 +570,20 @@ ifneq ($(SYSTEMD_FALLBACK_HOSTNAME),)
 SYSTEMD_CONF_OPTS += -Dfallback-hostname=$(SYSTEMD_FALLBACK_HOSTNAME)
 endif
 
+SYSTEMD_DEFAULT_TARGET = $(call qstrip,$(BR2_PACKAGE_SYSTEMD_DEFAULT_TARGET))
+ifneq ($(SYSTEMD_DEFAULT_TARGET),)
 define SYSTEMD_INSTALL_INIT_HOOK
-	ln -fs "$(call qstrip,$(BR2_PACKAGE_SYSTEMD_DEFAULT_TARGET))" \
+	ln -fs "$(SYSTEMD_DEFAULT_TARGET)" \
 		$(TARGET_DIR)/usr/lib/systemd/system/default.target
 endef
+SYSTEMD_POST_INSTALL_TARGET_HOOKS += SYSTEMD_INSTALL_INIT_HOOK
+endif
 
 define SYSTEMD_INSTALL_MACHINEID_HOOK
 	touch $(TARGET_DIR)/etc/machine-id
 endef
 
 SYSTEMD_POST_INSTALL_TARGET_HOOKS += \
-	SYSTEMD_INSTALL_INIT_HOOK \
 	SYSTEMD_INSTALL_MACHINEID_HOOK
 
 define SYSTEMD_INSTALL_IMAGES_CMDS