base: Handle backfilling in anonymous python code, not event handler

Handling of backfilling is trickier than you'd think. We need this to execute
early enough that the user will see the changes in bitbake -e and other output
yet late enough that the virtclass extensions have changed the tunes before
it executes.

It makes more sense to execute this at anonymous python time now bitbake -e
correctly handles this and that unbreaks multilib corruption of these variables.

[YOCTO #12373]

(From OE-Core rev: cefd312541019c1bb0132f41334df58971fd81ef)

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

meta/classes/base.bbclass

@@ -222,8 +222,6 @@ python base_eventhandler() {
         if not d.getVar("NATIVELSBSTRING", False):
             d.setVar("NATIVELSBSTRING", lsb_distro_identifier(d))
         d.setVar('BB_VERSION', bb.__version__)
-        oe.utils.features_backfill("DISTRO_FEATURES", d)
-        oe.utils.features_backfill("MACHINE_FEATURES", d)
         # Works with the line in layer.conf which changes PATH to point here
         setup_hosttools_dir(d.getVar('HOSTTOOLS_DIR'), 'HOSTTOOLS', d)
         setup_hosttools_dir(d.getVar('HOSTTOOLS_DIR'), 'HOSTTOOLS_NONFATAL', d, fatal=False)
@@ -382,6 +380,10 @@ def set_packagetriplet(d):
 python () {
     import string, re
 
+    # Handle backfilling
+    oe.utils.features_backfill("DISTRO_FEATURES", d)
+    oe.utils.features_backfill("MACHINE_FEATURES", d)
+
     # Handle PACKAGECONFIG
     #
     # These take the form: