Update U-Boot to current 2020.04 release for kirkwood platform.

Catch up with upstream and move some configuration options from
the header files to the corresponding defconfig files.

Compile tested: all devices
Run tested: nsa310, pogoplugv4

Tested-by: Cezary Jackiewicz <cezary@eko.one.pl> [nsa310]
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>