Port native tools to use KF5_HOST_TOOLING

The previous hack of just copying the native tools into the target sysroot
has several limitations:
- it no longer works with relative RPATHs being used
- it fails in the case of kpackage where we need the same tools for host
  and target

classes/kcoreaddons.bbclass

@@ -0,0 +1,13 @@
+# Class for recipes using kcoreaddons host tools
+
+do_compile_prepend() {
+    # desktoptojson needs to find installed service type files
+    export XDG_DATA_DIRS=${STAGING_DATADIR}:$XDG_DATA_DIRS
+}
+
+DEPENDS += " \
+    kcoreaddons \
+    kcoreaddons-native \
+"
+
+EXTRA_OECMAKE += " -DKF5_HOST_TOOLING=${STAGING_DIR_NATIVE}/${libdir}/cmake"