linaro: move the config fragment to kernel/configs

There is a new feature in 4.1, see:
63a91033: kbuild: add generic mergeconfig target, %.config

That allows config fragment to be dropped in kernel/configs or
arch/<ARCH>/configs.

Then, the fragment is merged by simply doing this command:

make <defconfig> <fragment>.config

This feature is very convenient and was backported in the 3.19 release branch, so
we can use it now, and get rid of the previous config fragment.

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>