On AArch64, ACPI must work with UEFI (EDK2). This way, the kernel is
always loaded from the disk image. We can not specify a direct custom
kernel while using ACPI.
To use a custom kernel, we have to replace the kernel file in the disk
image by:
- Making a copy of the Focal `raw` image
- Mounting the rootfs with `libguestfs-tools`
- Replacing the compressed kernel file
Signed-off-by: Michael Zhao <michael.zhao@arm.com>