Update the kernel version to 4.17.2.
Update U-Boot version to 2018.05.
Migrate to U-Boot generic distro configuration which is easier than the
hand written boot script.
Improve the 'dd' command example in readme.txt.
Cc: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
