enter-chroot: Bind mount /run/udev on newer versions of udev

udev control directory in Chromium OS >=6092 moved from `/dev/.udev` to `/run/udev`.

This commit makes sure it is accessible from the chroot (e.g. from X server).

In the chroot, udev shipped with precise/wheezy (version 175) is fine with just `/run/udev`, so there is no need to provide `/dev/.udev` as well.

Test: `sudo udevadm info --export-db | grep INPUT` should show some 40 lines of output.
Why not directly bindmount to /var/run/udev? We do it for /dev and /lib/modules...