test: allow multiple config options in buildconfigspec

test: allow multiple config options in buildconfigspec

In some cases we have alternative configuration options that supply the
same functionality, e.g CONFIG_NET and CONFIG_NET_LWIP.

Allow to specify all of them as arguments for buildconfigspec() and execute
the text if any of these is fulfilled, e.g.

    @pytest.mark.buildconfigspec('net', 'net_lwip')

Update the documentation.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>