babeld: Log to the system log instead of logging to a file

babeld: Log to the system log instead of logging to a file

This avoids running out of flash or RAM space if babeld logs too much.
The system log is stored on RAM by default and has a controlled size.

It is still possible to tell babeld to log to a file, by using the
following in /etc/config/babeld:

    config general
        option log_file "/var/log/babeld.log"

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