configs/wandboard: bump kernel and U-Boot versions

- Bump kernel to version 5.12.2.
- Bump U-Boot to version 2021.04.

While at it, switch U-Boot to the Kconfig build system and add some more
comments to the defconfig.

Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Cc: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>