net: syslog: Make sure CONFIG_LOG_IMMEDIATE is not set

The immediate logging option cannot be used with network logging
support CONFIG_LOG_BACKEND_NET as that would cause the generated
rsyslog messages to be malformed. The UDP packets would only have
one byte payloads which is not correct. So make sure that user is
not able to select a configuration with immediate mode and network
logging.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>