MLBEDSW-3402 SupportedOp now returns external name

Previously the internal operator type was printed when checking the
supported operator checks. This now converts that back to the external
type name.

Additionally removed dead code and changed the message for cpu-only ops

Signed-off-by: Michael McGeagh <michael.mcgeagh@arm.com>
Change-Id: Ib2b0cbcb49fdf63edb835828e266b079e63bae37
diff --git a/ethosu/vela/tflite_mapping.py b/ethosu/vela/tflite_mapping.py
index 8a53039..bbd44b0 100644
--- a/ethosu/vela/tflite_mapping.py
+++ b/ethosu/vela/tflite_mapping.py
@@ -689,3 +689,11 @@
 builtin_operator_inv_map = {v[0]: (k, v[1]) for k, v in builtin_operator_map.items()}
 
 builtin_operator_inv_map[Op.CustomNpuOp] = (BuiltinOperator.CUSTOM, CustomOptionsSerializer())
+
+
+def optype_to_builtintype(op_type):
+    if op_type in builtin_operator_inv_map:
+        builtin_type = builtin_operator_inv_map[op_type][0]
+        return next(k for k, v in vars(BuiltinOperator).items() if v == builtin_type)
+    else:
+        return "UNKNOWN"