efi: sandbox: Tidy up copy_fdt() to work with sandbox

efi: sandbox: Tidy up copy_fdt() to work with sandbox

At present this function takes a pointer as its argument, then passes this
to efi_allocate_pages(), which actually takes an address. It uses casts,
which are not supported on sandbox.

Also the function calculates the FDT size rounded up to the neared EFI
page size, then its caller recalculates the size and adds a bit more to
it.

This function is much better written as something that works with
addresses only, and returns both the address and the size of the relocated
FDT.

Also, copy_fdt() returns NULL on error, but really should propagate the
error from efi_allocate_pages(). To do this it needs to return an
efi_status_t, not a void *.

Update the code in this way, so that it is easier to follow, and also
supports sandbox.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>