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/README

@@ -1,3 +1,25 @@
+meta-gnome
+===========
+
+The recipe in this layer needs to have 'x11' in DISTRO_FEATURES 
+to have effect. To enable them, add in configuration file the following line.
+
+  DISTRO_FEATURES_append = " x11"
+
+If meta-gnome is included, but x11 is not enabled as a
+distro feature a warning is printed at parse time:
+
+    You have included the meta-gnome layer, but
+    'x11' has not been enabled in your DISTRO_FEATURES.
+
+If you know what you are doing, this warning can be disabled by setting the following
+variable in your configuration:
+
+  SKIP_META_GNOME_SANITY_CHECK = 1
+
+
+Dependencies
+------------
 This layer depends on:
 
 URI: git://github.com/openembedded/oe-core.git
@@ -14,3 +36,4 @@ When sending single patches, please using something like:
 'git send-email -M -1 --to openembedded-devel@lists.openembedded.org --subject-prefix=meta-gnome][PATCH'
 
 Interim layer maintainer: Martin Jansa <Martin.Jansa@gmail.com>
+