board: add support for Chromebook Snow

board: add support for Chromebook Snow

Chromebook Snow (Samsung XE303C12) is an Exynos 5 board with
a keyboard, an 11 inch screen and a battery attached.
It is relatively developer-friendly and can run mainline Linux
kernels with little to no effort.

There is barely anything special about this target as far as toolchain
is concerned, but its bootloader only accepts signed kernel images
in a Chromium OS specific format, and is not controllable otherwise.

This config provides a script for building the proper kernel blobs,
and a short manual for booting Buildroot images on this device.

In-tree exynos_defconfig is used for the kernel, with a fragment
to change mwifiex into a module. When built statically, mwifiex
attempts to load its firmware before rootfs is mounted and fails.

[Peter: use BR2_KERNEL_HEADERS_AS_KERNEL=y, lock kernel version,
	enable fit support in u-boot mkimage]
Signed-off-by: Alex Suykov <alex.suykov@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>