Get fallback from from file so it can be updated

This lets us update the fallback time via a script using a cron
job if we want to.

Author: cecille

build/chip/fallback_lkgt

@@ -0,0 +1 @@
+750275722

build/chip/write_build_time_header.py

@@ -18,6 +18,10 @@
 from datetime import datetime, timezone
 
 
+SCRIPT_DIR = os.path.abspath(os.path.dirname(__file__))
+FALLBACK_LKGT_FILENAME = os.path.abspath(os.path.join(SCRIPT_DIR, 'fallback_lkgt'))
+
+
 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
@@ -32,38 +36,48 @@ def __init__(self, output, define_name, define_val):
         self.define_val = define_val
 
 
-def GetOptions():
-    fallback_lkgt = datetime(2023, 10, 3, 0, 0, 0, 0, timezone.utc)
+def write_header(options):
+    with open(options.output, "w") as output_file:
+        output_file.write("// Generated by write_build_time_header.py\n")
+        output_file.write('#pragma once\n\n')
+
+        output_file.write(f'#define {options.define_name} {options.define_val}\n')
+
+
+def update_fallback_time_in_file():
+    with open(FALLBACK_LKGT_FILENAME, "w") as output_file:
+        output_file.write(str(utc_time_in_matter_epoch_s(datetime.now(tz=timezone.utc))))
 
+
+def main():
     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.")
+    parser.add_argument('--update-fallback-time-in-file', default=False, action='store_true',
+                        help='Write the current UTC time out to the fallback file')
     cmdline_options = parser.parse_args()
 
+    if cmdline_options.update_fallback_time_in_file:
+        update_fallback_time_in_file()
+        return
+
     # 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'
     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)
+        with open(FALLBACK_LKGT_FILENAME, "r") as input_file:
+            build_time = int(input_file.read())
 
-    return Options(output=output,
+    opts = Options(output=output,
                    define_name=define_name,
                    define_val=str(build_time))
+    write_header(opts)
 
 
-def WriteHeader(options):
-    with open(options.output, "w") as output_file:
-        output_file.write("// Generated by write_build_time_header.py\n")
-        output_file.write('#pragma once\n\n')
-
-        output_file.write(f'#define {options.define_name} {options.define_val}\n')
-
-
-options = GetOptions()
-WriteHeader(options)
+main()