Add DFU runtime and mode "class"

With the runtime and mode portions in separate classes, a single
application should only be building with one or the other enabled.  In
some applications both might be desired at build time.

The CFG_TUD_DFU_RUNTIME_AND_MODE option creates a DFU class, which asks
the application which mode to initialize to.  This allows a runtime
change between RT and DFU mode, by just reinitializing tusb.

src/tusb.h

@@ -100,6 +100,12 @@
     #include "class/dfu/dfu_mode_device.h"
   #endif
 
+  #if CFG_TUD_DFU_RUNTIME_AND_MODE
+    #include "class/dfu/dfu_rt_and_mode_device.h"
+    #include "class/dfu/dfu_rt_device.h"
+    #include "class/dfu/dfu_mode_device.h"
+  #endif
+
   #if CFG_TUD_NET
     #include "class/net/net_device.h"
   #endif