We should implement software gain. This improves user experience in two cases:
1. Default gain is too low
   a. board without capture gain control.
   b. board with capture gain control but the gain is still too low.
2. Let auto gain adjustment in Chrome app works.

Currently, the board that needs this is for https://code.google.com/p/chrome-os-partner/issues/detail?id=50139.

I checked the current capture gain control in Cras and found some issues:
1. Default system gain is 20dB, but that does not take effect if there is no identified capture gain control on the system.

2. On samus, 'Mic' affects both external mic and internal mic, but Cras does not recognize 'Mic' as a main capture control. Instead, Cras recognizes 'Mic' as a specific control and create a node for it, and leave that node as "unplugged", so it is not used. I guess there might be other boards having the same issue.

The above two issues do not bother us before because we generally set capture gain in UCM enable sequence and let it stay there. Also, we have not heard enough complaint about auto gain adjustment does not work.

Now I plan to add the software gain support and enable it only if "EnableSoftwareGain" is specified in UCM. This should unblock https://code.google.com/p/chrome-os-partner/issues/detail?id=50139.

After that, I should clean up the name matching issue with mandatory UCM so Cras can identify the correct capture gain control. Then, we can enable software gain on all the boards that really do not have capture gain control.