Replace thirdparty/GCS_MAVLink with a git submodule (#4556)

* Replace thirdparty/GCS_MAVLink with a git submodule

* Try to recover from missing submodules

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "radio/src/thirdparty/GCS_MAVLink/include_v1.0"]
+	path = radio/src/thirdparty/GCS_MAVLink/include_v1.0
+	url = https://github.com/mavlink/c_library_v1.git

radio/src/targets/common/avr/CMakeLists.txt

@@ -83,6 +83,17 @@ elseif(TELEMETRY STREQUAL JETI)
 elseif(TELEMETRY STREQUAL MAVLINK)
   add_definitions(-DTELEMETRY_MAVLINK)
   include_directories(${THIRDPARTY_DIR} gui/${GUI_DIR} targets/common/avr)
+  if(NOT EXISTS ${THIRDPARTY_DIR}/GCS_MAVLink/include_v1.0/mavlink_types.h)
+      message(WARNING "Warning: ${THIRDPARTY_DIR}/GCS_MAVLink/include_v1.0 is empty, trying to clone submodules")
+      execute_process(COMMAND git submodule update --init
+                              RESULT_VARIABLE GIT_SUBMODULE_RESULT
+                              WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR})
+      if(NOT ${GIT_SUBMODULE_RESULT} EQUAL 0)
+          message(FATAL_ERROR "Error: Cannot clone submodule at ${THIRDPARTY_DIR}/GCS_MAVLink/include_v1.0
+If you downloaded this project as a ZIP file, please instead run 
+'git clone --recursive https://github.com/opentx/opentx.git'")
+      endif()
+  endif()
   set(FIRMWARE_SRC
     ${FIRMWARE_SRC}
     targets/common/avr/telemetry_driver.cpp