mavlink: cmake generate mavlink headers at build time

- mavlink/mavlink is now directly included as a submodule instead of the generated mavlink/c_library_v2
 - this also switches to mavlink development.xml by default

src/modules/mavlink/mavlink_ftp.cpp

@@ -48,7 +48,7 @@
 #ifndef MAVLINK_FTP_UNIT_TEST
 #include "mavlink_main.h"
 #else
-#include <v2.0/standard/mavlink.h>
+#include <mavlink.h>
 #endif
 
 using namespace time_literals;