initmpfs: use initramfs if rootfstype= or root= specified

Command line option rootfstype=ramfs to obtain old initramfs behavior, and
use ramfs instead of tmpfs for stub when root= defined (for cosmetic
reasons).

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Rob Landley <rob@landley.net>
Cc: Jeff Layton <jlayton@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Stephen Warren <swarren@nvidia.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Jim Cromie <jim.cromie@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/ramfs-rootfs-initramfs.txt

@@ -79,6 +79,10 @@ to just make sure certain lists can't become empty.
 Most systems just mount another filesystem over rootfs and ignore it.  The
 amount of space an empty instance of ramfs takes up is tiny.
 
+If CONFIG_TMPFS is enabled, rootfs will use tmpfs instead of ramfs by
+default.  To force ramfs, add "rootfstype=ramfs" to the kernel command
+line.
+
 What is initramfs?
 ------------------