another stab at fixing out-of-tree build

diff --git a/applets/Kbuild b/applets/Kbuild
index 6c1c8a3..9830379 100644
--- a/applets/Kbuild
+++ b/applets/Kbuild
@@ -16,7 +16,7 @@
 
 # This trick decreases amount of rebuilds
 # if tree is merely renamed/copied
-ifeq ($(src),$(obj))
+ifeq ($(srctree),$(objtree))
 srctree_slash =
 else
 srctree_slash = $(srctree)/