task-guacamayo-openmax: new task that depends on MACHINE_FEATURE openmax

Whether we can support openmax is a machine feature rather than distro
feature; adding guacamayo task makes it possible to turn off the inclusion
of openmax for machines that support it.
raspberrypi.conf: added openmax to machine features
pulseaudio config: on RPi do not load the suspend-on-idle module

This is a work around for the crappy driver power management, see

https://github.com/raspberrypi/linux/issues/128