RAID autodetect has the side effect of requiring synchronisation
of all device drivers, which can make the boot several seconds longer
(I've measured 7 on one of my laptops).... even for systems that don't
have RAID setup for the root filesystem (the only FS where this matters).

This patch makes the default for autodetect a config option; either way
the user can always override via the kernel command line.
Signed-off-by: NArjan van de Ven <arjan@linux.intel.com>
Acked-by: NNeilBrown <neilb@suse.de>