babeld: Cleanup old compatibility code in initscript

babeld: Cleanup old compatibility code in initscript

It makes the init script more readable.  It has been more than two years
since babeld 1.5.1, let's hope nobody is still using the old
(undocumented) option names.

Incidentally, this commit fixes support for the "conf_dir" option,
introduced by ac643416dc ("babeld: allow changing alternative
configuration file and directory").  The default value of $OTHERCONFIGDIR
was always used, because the variable was used *before* it was (possibly)
redefined when parsing the UCI config.

Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>