busybox: mdev: Use mdev.seq

busybox: mdev: Use mdev.seq

/dev/mdev.seq exists to synchronize concurrently running instances of
mdev and to ensure that they execute in the proper order.  Without this
synchronization, it is possible to have inconsistent mount points, to
leak device nodes, or to have a node erroneously removed in rapid
hotplug scenarios.

Enable the use of mdev.seq by creating an empty /dev/mdev.seq at boot.

(From OE-Core rev: 3b256653ae9ab2cf56b21cb82a8bd2a562397b27)

Signed-off-by: Richard Tollerton <rich.tollerton@ni.com>
Signed-off-by: Ben Shelton <ben.shelton@ni.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>