py/makeversionhdr.py: Optionally get git tag and git hash from env vars.

This is handy when you are doing builds outside of the Git repository but
still want to record that information.

Signed-off-by: David Grayson <davidegrayson@gmail.com>

py/makeversionhdr.py

@@ -86,8 +86,11 @@ def get_version_info_from_mpconfig(repo_path):
 
 
 def make_version_header(repo_path, filename):
-    # Get version info using git, with fallback to py/mpconfig.h
-    info = get_version_info_from_git(repo_path)
+    info = None
+    if "MICROPY_GIT_TAG" in os.environ:
+        info = [os.environ["MICROPY_GIT_TAG"], os.environ["MICROPY_GIT_HASH"]]
+    if info is None:
+        info = get_version_info_from_git(repo_path)
     if info is None:
         info = get_version_info_from_mpconfig(repo_path)