linaro: add defconfig fragment with basic linaro config

In order to ease kernel configuration, we are now using kernel config
fragments. In the folder linaro/configs, we will add fragments as needed with
all configurations that are required when using the Linaro kernel releases.

Users are expected to run the following command to properly configure their
kernel tree prior to the build:

./scripts/kconfig/merge_config.sh arch/arm64/configs/defconfig \
                                  linaro/configs/distro.conf \
                                  [-O <build-dir>]

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