cbfstool: add ELF writing support

In order to generate rmodules in the format of ELF files
there needs to be support for writing out ELF files. The
ELF writer is fairly simple. It accpets sections that can
be associated with an optional buffer (file data). For each
section flagged with SHF_ALLOC a PT_LOAD segment is generated.
There isn't smart merging of the sections into a single PT_LOAD
segment.

Change-Id: I4d1a11f2e65be2369fb3f8bff350cbb28e14c89d
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/5377
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>