Remove unnecessary variable which sets
the RB_CDROM flag and causes the logic in vfs_mountroot_conf0
to probe acd0 and cd0 during boot.

In loader.conf, we specify the root device to mount from
with this:

vfs.root.mountfrom="cd9660:iso9660/TrueNAS_INSTALL"

When the system boots up, GEOM will probe all disk and CD devices, and
will make a GEOM label "iso9660/TrueNAS_INSTALL" available which will
be mounted.

If the "iso9660/TrueNAS_INSTALL" deivce is not available at boot time,
then this line in loader.conf:

vfs.mountroot_timeout="180"

will cause the kernel to wait 180 seconds for the device to appear.
This is necessary for booting off of IPMI Virtual CD-ROM devices,
which are slow to appear.

Removing  the RB_CDROM flag improves boot-time performance under
BHyve.

(cherry picked from commit 2e650393bf00b42906f09ffda0f65da66553f5eb)
Enable the /etc/netcli script to run on ttyu2, for BHyve.

BHyve does not emulate the video console, so does not have
/dev/ttyv[n] devices.  However, BHyve emulates a PCI serial port
on /dev/ttyu2.  Enable the /etc/netcli script to run on /dev/ttyu2,
so that the CLI menu can start on BHyve.
With this change, it is possible to boot and run *NAS under BHyve.

(cherry picked from commit bd4172ed514040ca72d64425b6b1870dbbb51718)