build: log warning on git too old for --update

build

@@ -556,6 +556,10 @@ Which components to build. Default: qemu-buildroot
                     # * https://stackoverflow.com/questions/2155887/git-submodule-head-reference-is-not-a-tree-error/25875273#25875273
                     # * https://github.com/boostorg/boost/issues/245
                     cmd.extend(['--depth', '1', LF])
+                else:
+                    self.log_warn('your git is too old for git submodule update --depth 1')
+                    self.log_warn('update to git 2.17 or newer and you will save clone time')
+                    self.log_warn('see: https://github.com/cirosantilli/linux-kernel-module-cheat/issues/44')
                 self.sh.run_cmd(cmd + ['--', LF] + submodule_ids_to_cmd(submodules_shallow))
 
         # Do the build.

common.py

@@ -1117,6 +1117,10 @@ lunch aosp_{}-eng
             if flush:
                 sys.stdout.flush()
 
+    def log_warn(self, msg):
+        with self.print_lock:
+            print('warning: {}'.format(msg), file=sys.stdout)
+
     def is_subpath(self, subpath, parent):
         '''
         https://stackoverflow.com/questions/3812849/how-to-check-whether-a-directory-is-a-sub-directory-of-another-directory