configs/stm32f469_disco: fix kernel bootup

configs/stm32f469_disco: fix kernel bootup

As described by [1], the kernel generated by the configuration for the
STM32f469 Discovery board is buggy. Using a newer kernel, as suggested
by [1], increases the dtb and Kernel image size. In particular, the
5.12 version of the kernel generates a dtb and a kernel image whose sum
exceeds the 2 MByte of the flash module.

So I decided to replace the afboot-stm32 bootloader in the flash with
U-boot to easily boot the system from sdcard without having to worry
about the size of dtb, kernel and rootfs generated by the configuration.

This solution allows you to fix the kernel boot issue and makes it
possible to use its future versions.

[1] http://buildroot-busybox.2317881.n4.nabble.com/Bug-11746-New-stm32f469-didn-t-work-correctly-td219644.html

Signed-off-by: Dario Binacchi <dariobin@libero.it>
Acked-by: Christophe Priouzeau <christophe.priouzeau@foss.st.com>
Tested-by: Christophe Priouzeau <christophe.priouzeau@foss.st.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[Arnout:
 - specify headers version explicitly, even though it's default;
 - bump kernel to 5.12.11]