Incorrect namespace dirs causing CRC check errors

[sam-bain]

I found out the hard way that you can't specify one of the library subdirectories (ie. DSDL/ardupilot/equipment/proximity_sensor) as one of your namespace dirs (as opposed to one of the base libraries, ie. DSDL/ardupilot).

When you do this, the generated message signature (ie. PROXIMITY_SENSOR_PROXIMITY_SIGNATURE) is different. This is used to calculate the CRC bits, which will be different if the two devices communicating are not using the same compilation.

Could be nice to have this fixed or clarified in the README :)