split up functions/scripts a bit. downloads avr-gcc

cmake/UpdateSubmodule.cmake

@@ -0,0 +1,22 @@
+# can be passed a relative path (to CMAKE_SOURCE_DIR) to update/checkout that submodule
+macro(update_submodule SUBMODULE)
+    find_package(Git QUIET)
+    if(GIT_FOUND AND EXISTS "${CMAKE_SOURCE_DIR}/.git")
+        option(GIT_SUBMODULE "Check submodules during build" ON)
+        if(GIT_SUBMODULE)
+            message(STATUS "Updating submoudle ${SUBMODULE}")
+            execute_process(COMMAND ${GIT_EXECUTABLE} submodule update ${SUBMODULE}
+                            WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
+                            RESULT_VARIABLE GIT_SUBMOD_RESULT)
+            if(NOT GIT_SUBMOD_RESULT EQUAL "0")
+                message(FATAL_ERROR "git submodule update ${SUBMODULE} failed with ${GIT_SUBMOD_RESULT}, please checkout submodules")
+            endif()
+        endif()
+    else()
+        message("Git not found - skipping submodule update")
+    endif()
+
+    if(NOT EXISTS "${CMAKE_SOURCE_DIR}/${SUBMODULE}/.git")
+        message(FATAL_ERROR "The submodule was not downloaded! GIT_SUBMODULE was turned off or failed. Please update submodules and try again.")
+    endif()
+endmacro()