linux: Import minimal defconfigs as defconfigs

Now that minimal kernel defconfigs are used in Buildroot, the problem
is that copying those minimal configuration files to .config in the
kernel source tree does not work, as kconfig will ask interactively
what should be the value for all unspecified options.

On suggestion on Sam Ravnborg, the easiest way to solve this is to
import the minimal defconfig file as a defconfig inside the kernel
tree (in arch/$(ARCH)/configs) and configure the kernel with it.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>