forward build-concurrency related env vars

This patch forwards environment variables to control the concurrency and
parallelism of the build to bitbake.
With that, the user can reduce the concurrency in case of running short
on memory. Further, by reducing the concurrency on very large and NUMA
machines, the build sometimes can be significantly speed up.

Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/context.py

@@ -105,7 +105,8 @@ class Context:
                             'No default locales set.', distro_bases)
 
         for key in ['http_proxy', 'https_proxy', 'ftp_proxy', 'no_proxy',
-                    'SSH_AUTH_SOCK']:
+                    'SSH_AUTH_SOCK',
+                    'BB_NUMBER_THREADS', 'PARALLEL_MAKE']:
             val = os.environ.get(key, None)
             if val:
                 self.environ[key] = val