Alsa support for Maemo SDK (n770): External Control plugin

Alsa support for Maemo SDK (n770): External Control plugin

This patch file adds an ALSA External Control plugin. This source uses
the dsp-protocol
implementation.

The plugin probes for all communication channel at the start time. It
will handle only
channels specified into alsa configuration file. An configuration example is:
        # Mixer
        ctl.!default {
                type dsp_ctl
                playback_devices ["/dev/dsptask/pcm2"]
                recording_devices ["/dev/dsptask/pcm_rec"]
        }

Signed-off-by: Eduardo Valentin <eduardo.valentin@indt.org.br>