util: add rmodtool for parsing ELF files to rmodules

The current implementation of creating rmodules relies
on invoking the linker in a certain manner with the
relocations overlaid on the BSS section. It's not really
surprising that the linker doesn't always behave the way
one wants depending on the linker used and the architecture.
Instead, introduce rmodtool which takes an ELF file as an
input, parses it, and creates a new ELF file in the format
the rmodule loader expects.

Change-Id: I31ac2d327d450ef841c3a7d9740b787278382bef
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/5378
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>