kas-container: track repo dir when using menu cmd

When using the kas menu command, we need to track the repository
location outside of the kas container as well. This is required to
prepare the environment for a later kas-container build, as the repo
path inside the container is different from the outside one. For that,
the location of KAS_REPO_DIR is passed via an env-var into the container.
There, this value is picked up by the menu plugin and written to the
.config.yaml file as `_source_host_dir`. When running kas-container build,
we check if the .config.yaml file contains this node and set the
KAS_REPO_DIR accordingly. The schema is extended accordingly.

Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
[Jan: rework _source_dir_host extraction to make it usable for menu as well]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/schema-kas.json

@@ -272,6 +272,9 @@
         },
         "_source_dir": {
             "type": "string"
+        },
+        "_source_dir_host": {
+            "type": "string"
         }
     }
 }