Use config variable (if defined) to specify remote.conf

Remote is specified in the configuration file by setting
BR2_XBMC_REMOTE_CONF to the file name of the file to be
copied to /etc/xbmc/remote.conf

The remote configuration file should be added to the
package/thirdparty/xbmc/etc/xbmc/ directory and use a
unique name that matches the specification in the file
used to configure the build.