efi/libstub: Take soft and hard memory limits into account for initrd loading

On x86, the preferred load address of the initrd is still below 4 GB,
even though in some cases, we can cope with an initrd that is loaded
above that.

To simplify the code, and to make it more straightforward to introduce
other ways to load the initrd, pass the soft and hard memory limits at
the same time, and let the code handling the initrd= command line option
deal with this.

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