x86/efi: Correct EFI boot stub use of code32_start

code32_start should point at the start of the protected mode code, and
*not* at the beginning of the bzImage. This is much easier to do in
assembly so document that callers of make_boot_params() need to fill out
code32_start.

The fallout from this bug is that we would end up relocating the image
but copying the image at some offset, resulting in what appeared to be
memory corruption.

Reported-by: Thomas Bächler <thomas@archlinux.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

diff --cc arch/x86/boot/compressed/eboot.c
Simple merge

diff --cc arch/x86/boot/compressed/head_32.S
Simple merge

diff --cc arch/x86/boot/compressed/head_64.S
Simple merge