builder: add bootstrap-chroot support for mock

fixes https://pagure.io/koji/issue/1017

builder/kojid

@@ -548,7 +548,13 @@ class BuildRoot(object):
         return koji.pathinfo.taskrelpath(self.task_id)
 
     def init(self):
-        rv = self.mock(['--init'])
+        args = ['--init']
+
+        if 'mock.bootstrap_chroot' in self.config['extra']:
+            if self.config['extra']['mock.bootstrap_chroot']:
+                args.append('--bootstrap-chroot')
+
+        rv = self.mock(args)
 
         if rv:
             self.expire()

docs/source/using_the_koji_build_system.rst

@@ -394,6 +394,9 @@ environment follows:
 * `mock.new_chroot` - 0/1 value. If it is set, `--new-chroot` or
   `--old-chroot` option is appended to any mock call. If it is not set,
   mock's default behavior is used.
+* `mock.bootstrap_chroot` - 0/1 value. If it is set, `--bootstrap-chroot`
+  is appended to the mock init call.  This tells mock to build in two stages,
+  using chroot rpm for creating the build chroot
 
 You may also specify per-tag environment variables for mock to use.
 For example, to set the CC environment variable to clang, you could