kconfig infra: support built-in config files

When the configuration file of a package is located inside of the
package sources, a make dependency can only be expressed after the
package has been extracted (and patched).

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>