nx: Set the NX compatible flag for the GRUB EFI images

nx: Set the NX compatible flag for the GRUB EFI images

For NX the GRUB binary has to announce that it is compatible with the
NX feature. This implies that when loading the executable GRUB image
several attributes are true:
  - the binary doesn't need an executable stack,
  - the binary doesn't need sections to be both executable and writable,
  - the binary knows how to use the EFI Memory Attributes Protocol on code
    it is loading.

This patch:
  - adds a definition for the PE DLL Characteristics flag GRUB_PE32_NX_COMPAT,
  - changes grub-mkimage to set that flag.

Signed-off-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Jan Setje-Eilers <jan.setjeeilers@oracle.com>
Signed-off-by: Mate Kukri <mate.kukri@canonical.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>