add support for alternative EasyBuild configuration options (WIP)

[boegel]

• let EasyBuild configuration parser accept alternative names for configuration options;
• make sure that determine value of a configuration option using alternative name works:
  • directly via self.options.build_path to get value for buildpath;
  • via build_option('build-path');
• handle situation where configuration option is specified using both old and new same (for example using --buildpath + $EASYBUILD_BUILD_PATH);