use un-def kernel by default

3.5.2-std-def-alt2 brings boot problems which were absent
with 3.4.x-std-def and are absent with 3.5.x-un-def;
seems like it's better to stay with known good variant
at the moment instead of having to fall back to it.

features.in/kernel/config.mk

@@ -1,4 +1,4 @@
 ###
 use/kernel:
 	@$(call add_feature)
-	@$(call set,KFLAVOURS,std-def)
+	@$(call set,KFLAVOURS,un-def)