fbt: reproducible manifest builds & improvements (#1801)

* fbt: reproducible manifest builds, less rebuild on small updates; scripts: assets: using timestamp from commandline af available
* fbt: added app import validation for launch_app & single app build targets
* fbt: COMSTR for app imports validation
* docs: minor fixes
* docs: markdown fix
* vscode: comments for RTOS startup

Co-authored-by: あく <alleteam@gmail.com>

site_scons/fbt/version.py

@@ -3,6 +3,11 @@ import datetime
 from functools import cache
 
 
+@cache
+def get_git_commit_unix_timestamp():
+    return int(subprocess.check_output(["git", "show", "-s", "--format=%ct"]))
+
+
 @cache
 def get_fast_git_version_id():
     try: