Added config file for volumio build.

This builds 3.8.13-bone20 compatible kernel for volumio. Need to use 
'LOCALVERSION=-bone20' when making the kernel and modules.
Remove 'DAC 'from 'IQaudIO DAC'

This is so the card name in alsa is 'IQaudIO' instead of 'DAC'
Fix volsw_range functions so SOC_DOUBLE_R_RANGE_TLV works.

This is so that the correct range of values as specified
with the SOC_DOUBLE_R_RANGE_TLV macro are sent to the
hardware for both the normal and invert cases.
Use a range macro for Playback Volume.

This allows limiting the output gain to avoid clipping in the
DAC ouput stages.