busybox: keep syslog.conf during sysupgrade

busybox: keep syslog.conf during sysupgrade

If a user finds that logd is too barebone for their needs and wishes
to have more control over syslog, the user presently has an option
to enable CONFIG_BUSYBOX_CONFIG_FEATURE_SYSLOG and configure syslog
with settings in /etc/syslog.conf.

Presently /etc/syslog.conf silently disappears on sysupgrade. This
patch prevents such unwanted behaviour if busybox syslog is enabled
via CONFIG_BUSYBOX_CONFIG_FEATURE_SYSLOG.

Signed-off-by: Val Kulkov <val.kulkov@gmail.com>