ncp-spinel: Add STREAM_MFG property for diagnostics module (#403)

It's a generic stream property which doesn't take care of the diagnostics feature details

  - get diags commands from the property with VALUE_SET command
  - return diags output to the property with VALUE_IS command

include/openthread-diag.h

@@ -43,6 +43,8 @@ void diagInit();
 
 char *diagProcessCmd(int argc, char *argv[]);
 
+char *diagProcessCmdLine(char *string);
+
 bool isDiagEnabled();
 
 #ifdef __cplusplus