Set USB PID based on kernel name

- Check uname for imx or mtk, and set the PID appropriately.
- Bail out if we don't match either.
- If we start to have multiple boards with the same kernel version,
we'll have to come up with a more clever way to handle this.

Change-Id: I98df4503646c23cc9f34fa9dcf027183c11c93a7