waf: add -Werror=type-limits

This catches always-true comparisons and the like

Also remove redundant directive

Tools/ardupilotwaf/boards.py

@@ -167,8 +167,8 @@ class Board:
             '-Werror=return-type',
             '-Werror=switch',
             '-Werror=sign-compare',
+            '-Werror=type-limits',
             '-Werror=unused-result',
-            '-Werror=return-type',
             '-Werror=shadow',
             '-Werror=unused-variable',
             '-Wfatal-errors',