Document build system targets. Add a rule to the BUILD module that is run if a source file does not exist to show an error to the user, rather than running the check-source rule before each build (slightly faster/more portable and the generated error is then a true make error).

LUFA/DoxygenPages/BuildSystem.txt

@@ -105,6 +105,10 @@
  *    <td><tt>mostlyclean</tt></td>
  *    <td>Remove all intermediatary files but preserve any binary output files.</td>
  *   </tr>
+ *   <tr>
+ *    <td><tt><filename>.s</tt></td>
+ *    <td>Create an assembly listing of a given input C/C++ source file.</td>
+ *   </tr>
  *  </table>
  *
  *  \section SSec_BuildModule_BUILD_MandatoryParams Mandatory Parameters