rmodtool: correct final memory size calculation

Apparently when I originally wrote this I confused myself to no end.
The code/data of an rmodule has a set memory size which is associated
with the .payload section. The relocation entries may increase the
overall footprint of the memory size if the rmodule has no bss but
a lot of relocations. Therefore, just compare relocation entries size
plus the file size of the .payload section with the memory size of the
paylod section. The .empty section is added only when we have not met
the final target size.

Change-Id: I5521dff048ae64a9b6e3c8f84a390eba37c7d0f5
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/6767
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)
Reviewed-by: Furquan Shaikh <furquan@google.com>