configs/odroidxu4: new defconfig

configs/odroidxu4: new defconfig

This patch provides support for the board Odroid XU4. It uses a recent
4.14 kernel released by Hardkernel, and the current u-boot 2017.05
also from Hardkernel.

Signed-off-by: Pierre CROKAERT <pct@crookies.net>
[Thomas:
 - Update DEVELOPERS file
 - Update .gitlab-ci.yml file
 - Simplify the boot.ini script
 - Add missing BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_14 option
 - Add BR2_PACKAGE_HOST_DOSFSTOOLS and BR2_PACKAGE_HOST_MTOOLS since a
   VFAT filesystem is generated
 - Keep the default root filesystem size, since it is sufficient.
 - Reduce the size of the VFAT filesystem]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>