scons: Consolidate the ProtoBuf code.

scons: Consolidate the ProtoBuf code.

The ProtoBuf support in src/SConscript was split into two parts, one
where the ProtoBuf sources were declared, and the other where scons was
told how to buld the .cc and .hh files and the .cc was added to the
build.

As far as I can tell, there was no real reason to have things split up
like that, at least not currently. This change moves everything into
the ProtoBuf class definition, and this should behave the same as
before but be a little easier to understand and maintain.

Change-Id: I02320f50ece53d90c14b5062bd6b1167210f46c3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/37275
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>