[ot] scripts/opentitan: pyot.py: emit a warning when unknown placeholder is detected

Signed-off-by: Emmanuel Blot <eblot@rivosinc.com>

Author: rivos-eblot

scripts/opentitan/pyot.py

@@ -593,6 +593,9 @@ def replace(smo: re.Match) -> str:
             name = smo.group(1)
             val = self._env[name] if name in self._env \
                 else environ.get(name, '')
+            if not val:
+                getLogger('pyot.file').warning("Unknown placeholder '%s'",
+                                               name)
             return val
         svalue = str(value)
         nvalue = re.sub(r'\$\{(\w+)\}', replace, svalue)