fff-babeld: Add prefix to configuration name

When the user removes a peer from /etc/config/gateway,
the script has to be able to find any leftover configurations
that have been created for it.

This can be made very easy by prepending a prefix to every
automatically generated configuration.

This also allows to remove the nasty check for the babeld default interface.
Therefore the naming check for cfg* also can be removed.

As a side effect all manually created configuration blocks that do not match a
prefix will not be touched by configuregateway.

Signed-off-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Robert Langhammer <rlanghammer@web.de>