of: Fix premature bootconsole disable with 'stdout-path'

Support for devicetree serial consoles via 'stdout-path' causes
bootconsoles to be disabled when the vt dummy console loads, since
there is no preferred console (the preferred console is not added
until the device is probed).

Ensure there is at least a preferred console, even if never matched.

Requires: "console: Fix console name size mismatch"
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Rob Herring <robh@kernel.org>