kunit: tool: Add support for overriding the QEMU serial port

On some platforms, the console is not the first serial port.  To make
this work, the first serial port in QEMU must be set to "null".

Add support for this by adding an optional "serial" parameter, which
defaults to "stdio", and can be overridden by platform-specific
configuration.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>