top: Enable Ruff linter to check undefined-name (F821).

Also adds some global ignores for manifest files (which have implicit
imports) and the multitests (which have the same).

Other F821 fixes or accommodations are in the parent commits to this
commit.

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

pyproject.toml

@@ -62,7 +62,6 @@ ignore = [
   "F403",
   "F405",
   "F541",
-  "F821",
   "F841",
   "ISC003",  # micropython does not support implicit concatenation of f-strings
   "PIE810",  # micropython does not support passing tuples to .startswith or .endswith
@@ -91,3 +90,10 @@ max-statements = 166
 
 [tool.ruff.per-file-ignores]
 "micropython/aiorepl/aiorepl.py" = ["PGH001"]
+
+# manifest.py files are evaluated with some global names pre-defined
+"**/manifest.py" = ["F821"]
+"ports/**/boards/manifest*.py" = ["F821"]
+
+# ble multitests are evaluated with some names pre-defined
+"micropython/bluetooth/aioble/multitests/*" = ["F821"]