rapidio: add modular build option for the subsystem core

Add a configuration option to build RapidIO subsystem core code as a
loadable kernel module.  Currently this option is available only for
x86-based platforms, with the additional patch for PowerPC planned to be
provided later.

This patch replaces kernel command line parameter "riohdid=" with its
module-specific analog "rapidio.hdid=".

Signed-off-by: Alexandre Bounine <alexandre.bounine@idt.com>
Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: Li Yang <leoli@freescale.com>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: Andre van Herk <andre.van.herk@Prodrive.nl>
Cc: Micha Nelissen <micha.nelissen@Prodrive.nl>
Cc: Stef van Os <stef.van.os@Prodrive.nl>
Cc: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>