santiy-meta-gnome: add class for bbappend files checking

Idea taken from meta-virt

Add a new class, sanity-meta-gnome.bbclass, to check for whether necessary
settings are available for bbappend files in this layer to be effective,
and warn users if not.

In addition, a variable SKIP_SANITY_BBAPPEND_CHECK is added to enable users
to explicitly skip the checking to avoid unwanted warnings.

Signed-off-by: Armin Kuster <akuster808@gmail.com>

meta-gnome/classes/sanity-meta-gnome.bbclass

@@ -0,0 +1,10 @@
+addhandler gnome_bbappend_distrocheck
+gnome_bbappend_distrocheck[eventmask] = "bb.event.SanityCheck"
+python gnome_bbappend_distrocheck() {
+    skip_check = e.data.getVar('SKIP_META_GNOME_SANITY_CHECK') == "1"
+    if 'x11' not in e.data.getVar('DISTRO_FEATURES').split() and not skip_check:
+        bb.warn("You have included the meta-gnome layer, but \
+'x11' has not been enabled in your DISTRO_FEATURES. Some bbappend files \
+may not take effect. See the meta-gnome README for details on enabling \
+meta-gnome support.")
+}