configs/qemu_arm_vexpress_tz: bump versions and update

configs/qemu_arm_vexpress_tz: bump versions and update

- Bump TF-A to version 2.4.
- Bump u-boot to version 2021.01.
- Bump kernel to version 5.11.3.

We switch TF-A to a single FIP image. Thanks to this, TF-A does not need to
use semihosting to load the various BL* anymore (but U-Boot still does).
Update the readme.txt accordingly.

We switch to a u-boot image for the ramdisk. This removes the need to
update the fdt chosen node manually in the bootcmd.

While at it, we drop the generation of the kernel dtb, which we do not use.
In this config, we are indeed using the dtb generated on-the-fly by qemu
and amended by TF-A.

Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Cc: Gerome Burlats <gerome.burlats@smile.fr>
Cc: Romain Naour <romain.naour@gmail.com>
Cc: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Tested-by: Romain Naour <romain.naour@gmail.com>
Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>