configs/aarch64_efi: bump kernel version and update

configs/aarch64_efi: bump kernel version and update

- Bump kernel to version 5.11.16.

We remove the hardcoded ttyAMA0 and rely on the firmware to discover our
console. This enables serial console on systems, which do not have an Arm
pl011 UART.

We switch to GPT disklabel and discover our root filesystem using its
PARTLABEL. This enables booting from more media, such as HDD, SD card or
USB.

We update the readme, which hinted that ACPI was mandatory. This is not
strictly the case as we can also boot with a dtb and/or a U-Boot based
firmware, with no ACPI. While at it, mention EBBR, SystemReady and explain
how to build and use a U-Boot-based qemu firmware.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Erico Nunes <nunes.erico@gmail.com>
Reviewed-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>