Makefile: ignore .config for defconfig

Currently, we do not include .config for all '%_defconfig' targets, but
we forgot to also exclude plain 'defconfig'.

Reported-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>