efi: Add a comment about duplicated ELF constants

These constants are defined in arch-specific code but redefined here. Add
a TODO to clean this up.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>