bitbake: utils: only add layer once in edit_bblayers_conf()

Prevent edit_bblayers_conf() from adding layer(s) multiple times. This
happened when BBLAYERS variable was "listed" multiple times in
bblayer.conf - i.e. the configuration was split into multiple separate
assignments.

[YOCTO #8316]

(Bitbake rev: 5e423237f9f4ff7e7e03bf066b0142ba4bd82219)

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/utils.py

@@ -1249,6 +1249,7 @@ def edit_bblayers_conf(bblayers_conf, add, remove):
                     bblayers.append(addlayer)
                 else:
                     notadded.append(addlayer)
+            del addlayers[:]
 
         if updated:
             return (bblayers, None, 2, False)