efi_loader: Fix memory corruption on 32bit systems

It's pretty unlikely that anyone is going to be using EFI authentication
on a 32bit system.  However, if you did, the efi_prepare_aligned_image()
function would write 8 bytes of data to the &efi_size variable and it
can only hold 4 bytes so that corrupts memory.

Fixes: 163a0d7e ("efi_loader: add PE/COFF image measurement")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>