util/mkimage: Improve data_size value calculation

util/mkimage: Improve data_size value calculation

According to "Microsoft Portable Executable and Common Object File Format
Specification", the Optional Header SizeOfInitializedData field contains:

  Size of the initialized data section, or the sum of all such sections if
  there are multiple data sections.

Make this explicit by adding the GRUB kernel data size to the sum of all
the modules sizes. The ALIGN_UP() is not required by the PE spec but do
it to avoid alignment issues.

Signed-off-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>