Revert "prevent recursion in %_defconfig rules"

This reverts commit 94dd02f5.

The change breaks defconfigs from BR2_EXTERNAL, both for in-tree and
out-of-tree builds.

Besides, the problem reported in 94dd02f5 could not be reproduced.

I can read French, and I suspect a relative path was used for either
BR2_EXTERNAL or O.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Jérémy Rosen <jeremy.rosen@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>