Add morph cross-bootstrap

Cross-bootstrap is a way to build baserock on an architecture that
does not currently have Baserock. It can be used by `morph
cross-bootstrap <ARCH> <REPO> <REF> <MORPH>`, and will build an artifact
that can be used as a root filesystem with a basic build environment
with a script named `native-bootstrap` which will build and install
every chunk in the system.

If done with a devel system, this will give you a suitable environment
for building a proper Baserock system.

This does not currently provide a kernel for the target architecture.

Apart from adding the cross-bootstrap plugin, it also makes the
following changes:

* Moves the lit of valid_archs into morphlib (instead of locally-scoped
  in MorphologyFactory)
* BuildCommand takes an extra argument, build_env
* split BuildCommand's get_artifact_object into create_source_pool and
  resolve_artifacts (plus changes things that use get_artifact_object to
  use the new way)
* setup_mounts finds out whether to do so by whether build_mode is
 'staging', instead of by whether the setting 'staging-chroot' is true.
* Makes ChunkBuilder's get_sources use the
  morphlib.builder2.extract_sources() method, and moved
  set_mtime_recursively into morphlib.builder2, since it's not currently
  used anywhere else.
* moved ChunkBuilder's get_commands into the Morphology class (plus
  changes to anything that used get_commands)

1 files changed, 4 insertions, 2 deletions

diff --git a/morphlib/plugins/graphing_plugin.py b/morphlib/plugins/graphing_plugin.py
index 75514eec..b57a3f3e 100644
--- a/morphlib/plugins/graphing_plugin.py
+++ b/morphlib/plugins/graphing_plugin.py
@@ -65,7 +65,9 @@ class GraphingPlugin(cliapp.Plugin):
                                 '%(repo_name)s %(ref)s %(filename)s',
                             repo_name=repo_name, ref=ref, filename=filename)
             builder = morphlib.buildcommand.BuildCommand(self.app)
-            artifact = builder.get_artifact_object(repo_name, ref, filename)
+            srcpool = build_command.create_source_pool(repo_name, ref,
+                                                       filename)
+            root_artifact = build_command.resolve_artifacts(srcpool)
 
             basename, ext = os.path.splitext(filename)
             dot_filename = basename + '.gv'
@@ -81,7 +83,7 @@ class GraphingPlugin(cliapp.Plugin):
 
             with open(dot_filename, 'w') as f:
                 f.write('digraph "%s" {\n' % basename)
-                for a in artifact.walk():
+                for a in root_artifact.walk():
                     f.write(
                         '  "%s" [shape=%s];\n' %
                         (a.name,