We never use any of these other drivers, so having them around just takes
up space.

Change-Id: Iced812162df1fef3fd55522b7e700acb6c3bcd41

Using the coprocessor was a good idea in theory, but didn't actually work.
This moves all SDL threads to the main core, which isn't ideal, but at
least works. Additionally, this also adds some good-practice stuff such as
setting the default sample rate on exit.

Change-Id: If1636b33d439000ec7c4e50f46e8d002708d3121

This prevents a (highly improbable) race condition when exiting, and uses
pcm_set_frequency() instead of mixer_set_frequency(), since that seems like
the Right Thing to Do (TM).

Change-Id: Icb6a4597843215f08b3835490ac63f67c9a04736

This ports Fabien Sanglard's Chocolate Duke to run on a version of SDL
for Rockbox.

Change-Id: I8f2c4c78af19de10c1633ed7bb7a997b43256dd9