core-image-minimal-xfce: Use graphical.target as default

This ensures that nodm unit is started during boot with systemd images

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Acked-by: Kai Kang <kai.kang@windriver.com>

meta-xfce/recipes-core/images/core-image-minimal-xfce.bb

@@ -16,3 +16,5 @@ LICENSE = "MIT"
 export IMAGE_BASENAME = "core-image-minimal-xfce"
 
 inherit core-image
+
+SYSTEMD_DEFAULT_TARGET = "graphical.target"