cloud-hypervisor/edk2 publishes prebuilt CLOUDHV.fd (x86-64) and
CLOUDHV_EFI.fd (AArch64) as release assets. docs/uefi.md only
described the build from source, and the AArch64 firmware
customizations required for cloud-hypervisor were left undocumented.
Add a "Using Prebuilt UEFI Firmware" section to docs/uefi.md and
an "AArch64 Firmware Notes" section covering both customizations.
Updates to "Building UEFI Firmware for AArch64" section.
Switch the boot examples from --kernel to --firmware, which is the
direct UEFI load path on AArch64.
Minor README.md updates.
Signed-off-by: Saravanan D <saravanand@crusoe.ai>