configs/pandaboard: bump kernel and U-Boot versions

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

While at it, enable VFPv3 with 32 registers (instead of 16) and add a few
comments to the defconfig.

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