efi/zboot: arm64: Poke kernel code size into the zboot payload image header

The EFI zboot code is not built as part of the kernel proper, like the
ordinary EFI stub, but still needs access to symbols that are defined
only internally in the kernel, and are left unexposed deliberately to
avoid creating ABI inadvertently that we're stuck with later.

So instead of passing the ordinary Image file to the zboot make rules,
create an alternate version Image.zboot that has the code size copied
into the header into a field that has meaning in the bare metal boot
ABI, but is actually not used anymore, and is always set to 0x0.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>