udev: only use devtmpfs for udev

udev: only use devtmpfs for udev

Fix for bug: https://bugzilla.yoctoproject.org/show_bug.cgi?id=4632
Since udev 172, the 'mknod' logic was removed from udev. Yocto Dylan
is now using udev 182. This means /dev is now required to be a
devtmpfs filesystem (maintained by the kernel).    If the root
filesystem is a ramdisk,  the kernel's auto-mount of /dev doesn't
activate since there is no rootfs to actually mount...  The bug causes
an unusable system as /dev doesn't contain even basic nodes required
to even get a login prompt.

The Yocto  udev/init script mounts tmpfs if it does not detect tmpfs
or devtmpfs mounted at /dev.  This appears to be  outdated logic that
is no longer correct. I believe the Yocto udev init script should be
checking and mounting only 'devtmpfs' on dev.

(From OE-Core rev: 31ab19ab69bc6504df01cac7ee0670ca78d247ab)

Signed-off-by: Alex Olson <alex.olson+yocto@gmail.com>
[YOCTO #4632]

Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>