system: default to devtmpfs for /dev

As discussed on the list, devtmpfs is quite a bit nicer default than
the old static /dev, so change it.

Notice that you NEED to enable CONFIG_DEVTMPFS / CONFIG_DEVTMPFS_MOUNT
in the kernel configuration if you manually build a kernel outside
buildroot, otherwise the rootfs won't work.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>