Add build flag for changing firmware build time

Adding a default fallback firmware build time. This makes the
firmware build more consistent, so you get the same binary unless
you specifically ask for the build time to be updated.

build/chip/write_build_time_header.py

@@ -18,10 +18,10 @@
 from datetime import datetime, timezone
 
 
-def utc_time_in_matter_epoch_s():
+def utc_time_in_matter_epoch_s(time: datetime):
     """ Returns the time in matter epoch in s. """
     # Matter epoch is 0 hours, 0 minutes, 0 seconds on Jan 1, 2000 UTC
-    utc_matter = datetime.now(tz=timezone.utc) - datetime(2000, 1, 1, 0, 0, 0, 0, timezone.utc)
+    utc_matter = time - datetime(2000, 1, 1, 0, 0, 0, 0, timezone.utc)
     return int(utc_matter.total_seconds())
 
 
@@ -33,17 +33,24 @@ def __init__(self, output, define_name, define_val):
 
 
 def GetOptions():
+    fallback_lkgt = datetime(2023, 10, 3, 0, 0, 0, 0, timezone.utc)
+
     parser = argparse.ArgumentParser()
     parser.add_argument('--output', help="Output header name (inside gen dir)")
     parser.add_argument('--gen-dir',
                         help="Path to root of generated file directory tree.")
+    parser.add_argument('--use-current-time', default=False, action='store_true',
+                        help="Set the LKGT to the current time. If this flag is not used, the LKGT is set to a hardcoded time.")
     cmdline_options = parser.parse_args()
 
     # The actual output file is inside the gen dir.
     output = os.path.join(cmdline_options.gen_dir, cmdline_options.output)
 
     define_name = 'CHIP_DEVICE_CONFIG_FIRMWARE_BUILD_TIME_MATTER_EPOCH_S'
-    build_time = utc_time_in_matter_epoch_s()
+    if cmdline_options.use_current_time:
+        build_time = utc_time_in_matter_epoch_s(datetime.now(tz=timezone.utc))
+    else:
+        build_time = utc_time_in_matter_epoch_s(fallback_lkgt)
 
     return Options(output=output,
                    define_name=define_name,

src/credentials/BUILD.gn

@@ -20,6 +20,7 @@ import("${chip_root}/src/platform/device.gni")
 
 declare_args() {
   chip_build_example_creds = true
+  update_last_known_good_time = false
 }
 
 action("gen_build_time_header") {
@@ -35,6 +36,9 @@ action("gen_build_time_header") {
     "--gen-dir",
     rebase_path(include_dir, root_build_dir),
   ]
+  if (update_last_known_good_time) {
+    args += [ "--use-current-time" ]
+  }
 
   visibility = [ ":build_time_header" ]
 }