buildroot

linux: migrate to the kconfig infrastructure

Migrate the linux package to the kconfig infrastructure.
A notable change compared to the original behavior:

- the targets linux-update-(def)config are now always saving the config
  file, even for a defconfig bundled in the linux sources. This is done
  to keep the kconfig infrastructure simple.

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>