x86/efi: Fix code32_start value in EFI boot stub

The correct value for code32_start isn't image->image_base because that
address points to the real mode sectors, and so doesn't point to
startup_32. Instead compute the actual address of startup_32 in
efi_pe_entry() and assign to code32_start.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>