[RFC PATCH 3/3] base.bbclass: Add detection/sanity check of non-global class inheritance

[Richard Purdie <richard.purdie@linuxfoundation.org>]

Users are doing things like inheriting bootimg in local.conf and not understanding
that there are two types of class, "global" classes and "recipe" classes. This
patch starts to markup the recipe classes to tell the user when they're including
a "recipe" class in global context by mistake.

RFC: Only one class has markup here but we could go through them and mark the "recipe"
classes accordingly.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
---
diff --git a/meta/classes/base.bbclass b/meta/classes/base.bbclass
index 360b112..08617e8 100644
--- a/meta/classes/base.bbclass
+++ b/meta/classes/base.bbclass
@@ -295,6 +295,9 @@ python base_eventhandler() {
         preferred_ml_updates(e.data)
         oe.utils.features_backfill("DISTRO_FEATURES", e.data)
         oe.utils.features_backfill("MACHINE_FEATURES", e.data)
+        nonglobalclasses = e.data.getVar("NONGLOBALBBCLASS", True)
+        if nonglobalclasses:
+            bb.error("The following classes were inserted into the global namespace but are not designed for use like this. Please inherit them in the appropriate recipe instead: %s" % nonglobalclasses)
 
     if isinstance(e, bb.event.BuildStarted):
         localdata = bb.data.createCopy(e.data)
diff --git a/meta/classes/bootimg.bbclass b/meta/classes/bootimg.bbclass
index b13eef9..1562408 100644
--- a/meta/classes/bootimg.bbclass
+++ b/meta/classes/bootimg.bbclass
@@ -4,6 +4,8 @@
 # Creates a bootable image using syslinux, your kernel and an optional
 # initrd
 
+NONGLOBALBBCLASS += "bootimg"
+
 #
 # End result is two things:
 #