efi: libstub: pass image handle to handle_kernel_image()

In a future patch, arm64's implementation of handle_kernel_image() will
omit randomizing the placement of the kernel if the load address was
chosen randomly by the loader. In order to do this, it needs to locate a
protocol on the image handle, so pass it to handle_kernel_image().

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