[Buildroot] mdev handling

[Yegor Yefremov]

>> What should "Using devtmfs" make? Enable/disable this option in
>> kernel?
>
> Yes, probably, and make sure that devtmpfs is mounted as needed
> (/etc/fstab).

Does it have to be mounted? It is made by the kernel just after
mounting the rootfs (at least there is such a kernel option):

CONFIG_DEVTMPFS=y
CONFIG_DEVTMPFS_MOUNT=y

So our initial /etc/fstab is sufficient for this purpose.

Yegor