osbuild.py/pipeline: assign a unique id to every pipeline

This uniquely identifies a pipeline based on its content. Pipelines
are considered equal modulo whitespace and the order of object
elements.

The intention is that two runs of a pipeline with the same id
generates functionaly equivalent ids. It is up to the writers
of stages and pipelines to ensure this property holds.

Signed-off-by: Tom Gundersen <teg@jklm.no>

osbuild.py

@@ -1,4 +1,5 @@
 
+import hashlib
 import json
 import os
 import subprocess
@@ -203,6 +204,10 @@ def print_header(title, options, machine_name):
 
 class Pipeline:
     def __init__(self, pipeline):
+        m = hashlib.sha256()
+        m.update(json.dumps(pipeline, sort_keys=True).encode())
+
+        self.id = m.hexdigest()
         self.stages = pipeline["stages"]
         self.assembler = pipeline.get("assembler")