efi/libstub: Pass loaded image EFI handle to efi_load_initrd()

In preparation for allowing loaders to pass the initrd loadfile2
protocol implementation via the loaded image handle instead of a global
singleton device path, pass the loaded image EFI handle down to the code
that loads the initrd.

Tested-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>