kconfig infra: add support for defconfig files

This commit adds support for using a defconfig file instead of a full
.config. This is a precondition to migrate packages like linux and
barebox to the kconfig infrastructure.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: "Yann E. Morin" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>