Make devtmpfs available on (embedded) configurations without SHMEM/TMPFS,
using ramfs instead.

Saves ~15KB.
Signed-off-by: NPeter Korsgaard <jacmet@sunsite.dk>
Acked-by: NKay Sievers <kay.sievers@vrfy.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>