Add support for DFU Runtime class for devices

This is really just a few descriptors and then answering to the
request from the host to reboot into DFU mode.

That latter part is delegated to the app since this is platform
specific.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>

src/tusb_option.h

@@ -188,6 +188,10 @@
   #define CFG_TUD_USBTMC          0
 #endif
 
+#ifndef CFG_TUD_DFU_RT
+  #define CFG_TUD_DFU_RT          0
+#endif
+
 
 //--------------------------------------------------------------------
 // HOST OPTIONS