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, 3 insertions, 4 deletions

diff --git a/morphlib/morphologyfactory.py b/morphlib/morphologyfactory.py
index 7f5cd94a..d0d9ea04 100644
--- a/morphlib/morphologyfactory.py
+++ b/morphlib/morphologyfactory.py
@@ -106,15 +106,14 @@ class MorphologyFactory(object):
                                  '(arch is a mandatory field)' %
                                  filename)
 
-        valid_archs = ['armv7l', 'armv7b', 'x86_32', 'x86_64']
-
         if morphology['arch'] == 'armv7':
             morphology._dict['arch'] = 'armv7l'
 
-        if morphology['arch'] not in valid_archs:
+        if morphology['arch'] not in morphlib.valid_archs:
             raise morphlib.Error('Unknown arch %s. This version of Morph '
                                  'supports the following architectures: %s' %
-                                 (morphology['arch'], ', '.join(valid_archs)))
+                                 (morphology['arch'],
+                                  ', '.join(morphlib.valid_archs)))
 
         kind = morphology['system-kind']
         if kind == 'rootfs-tarball': # pragma: no cover