py/makeversionhdr: Honor SOURCE_DATE_EPOCH if present.

This environment variable, if defined during the build process,
indicates a fixed time that should be used in place of "now" when
such a time is explicitely referenced.

This allows for reproducible builds of micropython.
See https://reproducible-builds.org/specs/source-date-epoch/Signed-off-by: iTitou <moiandme@gmail.com>

py/makeversionhdr.py

@@ -80,6 +80,12 @@ def make_version_header(filename):
 
     git_tag, git_hash = info
 
+    build_date = datetime.date.today()
+    if "SOURCE_DATE_EPOCH" in os.environ:
+        build_date = datetime.datetime.utcfromtimestamp(
+            int(os.environ["SOURCE_DATE_EPOCH"])
+        ).date()
+
     # Generate the file with the git and version info
     file_data = """\
 // This file was generated by py/makeversionhdr.py
@@ -89,7 +95,7 @@ def make_version_header(filename):
 """ % (
         git_tag,
         git_hash,
-        datetime.date.today().strftime("%Y-%m-%d"),
+        build_date.strftime("%Y-%m-%d"),
     )
 
     # Check if the file contents changed from last time