Add meta-config

* new local.conf file can contain multiple config file settings to be
  merged in to existing project config files
* localrc can be embedded into local.conf and will auto-extract if
  localrc does not exist
* Adds functions get_meta_section_files(), get_meta_section(),
  merge_config_file() and merge_config_group()

Change-Id: I367cadc86116621e9574ac203aafdab483d810d3
Merge branch 'config-local' into dt-test