Close #326: Implement command to retrieve and set configuration values (#359)

* Implement command to retrieve and set configuration values
* qemu: show advice to use "pmbootstrap config"
* Allow "pmbootstrap config" without positional arguments (prints the full config)
* Check valid variable names

pmb/helpers/logging.py

@@ -101,3 +101,8 @@ def init(args):
     log_handler._args = args
     handler.setFormatter(formatter)
     root_logger.addHandler(handler)
+
+
+def disable():
+    logger = logging.getLogger()
+    logger.disabled = True