top: Enable ruff linter check for F821 undefined-name.

Very helpful for catching typos or missing imports when writing code!

Description can be found at
https://beta.ruff.rs/docs/rules/undefined-name/

Parent commits contain various small fixes and inline ignores for this
check.  The only blanket exception is manifest files, which are numerous
and evaluated with some global names pre-defined - these can be globally
ignored.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

pyproject.toml

@@ -35,7 +35,6 @@ ignore = [
   "F401",
   "F403",
   "F405",
-  "F821",
   "PLC1901",
 ]
 line-length = 337
@@ -46,3 +45,7 @@ max-complexity = 40
 
 [tool.ruff.per-file-ignores]
 "ports/cc3200/tools/uniflash.py" = ["E711"]
+
+# manifest.py files are evaluated with some global names pre-defined
+"**/manifest.py" = ["F821"]
+"ports/**/boards/manifest*.py" = ["F821"]