MLBEDSW-839: Code generation using external API

Added external API to generate register command streams.

Existing code generation has been refactored to make
use of this API.

Change-Id: Ibb4c2b167809869f16470b14da24f08a65c82b7b
Signed-off-by: Louis Verhaard <louis.verhaard@arm.com>
diff --git a/ethosu/vela/tflite_reader.py b/ethosu/vela/tflite_reader.py
index 24f9f87..b3b0720 100644
--- a/ethosu/vela/tflite_reader.py
+++ b/ethosu/vela/tflite_reader.py
@@ -23,6 +23,7 @@
 from .errors import TensorError
 from .nn_graph import Graph
 from .nn_graph import Subgraph
+from .operation import create_activation_function
 from .operation import Op
 from .operation import Operation
 from .tensor import QuantizationParameters
@@ -186,7 +187,9 @@
             if "depth_multiplier" in op.attrs:
                 op.attrs["channel_multiplier"] = op.attrs["depth_multiplier"]
 
-            op.activation = op.attrs.pop("fused_activation_function", None)
+            faf = op.attrs.pop("fused_activation_function", None)
+            if faf is not None:
+                op.activation = create_activation_function(faf)
             if custom_code is not None:
                 op.attrs["custom_code"] = custom_code