base: Add deferred class event handler

Use the new deferred class event to set the class overrides earlier.
This improves interaction of the override with PACKAGECONFIG values
that control conditional inherits (such as python support).

This also allows toolchain configuration in an easier and more user
friendly way.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

Author: rpurdie

meta/classes-global/base.bbclass

@@ -267,10 +267,19 @@ def buildcfg_neededvars(d):
         bb.fatal('The following variable(s) were not set: %s\nPlease set them directly, or choose a MACHINE or DISTRO that sets them.' % ', '.join(pesteruser))
 
 addhandler base_eventhandler
-base_eventhandler[eventmask] = "bb.event.ConfigParsed bb.event.MultiConfigParsed bb.event.BuildStarted bb.event.RecipePreFinalise bb.event.RecipeParsed"
+base_eventhandler[eventmask] = "bb.event.ConfigParsed bb.event.MultiConfigParsed bb.event.BuildStarted bb.event.RecipePreFinalise bb.event.RecipeParsed bb.event.RecipePreDeferredInherits"
 python base_eventhandler() {
     import bb.runqueue
 
+    if isinstance(e, bb.event.RecipePreDeferredInherits):
+        # Use this to snoop on class extensions and set these up before the deferred inherits
+        # are processed which allows overrides on conditional variables.
+        if "native" in e.inherits:
+            d.setVar('CLASSOVERRIDE', 'class-native')
+        elif "nativesdk" in e.inherits:
+            d.setVar('CLASSOVERRIDE', 'class-nativesdk')
+        return
+
     if isinstance(e, bb.event.ConfigParsed):
         if not d.getVar("NATIVELSBSTRING", False):
             d.setVar("NATIVELSBSTRING", lsb_distro_identifier(d))