Bump U-Boot and kernel to their latest releases.
Remove the documented limitations. Recent kernels added support for the
SOM SPI flash, and the carrier SFP port.
Mention the 'dd' command in the warning text. No script is involved.
Cc: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
