net: Convert core IP stack to use log levels

Instead of one global log level option and one on/off boolean
config option / module, this commit creates one log level option
for each module. This simplifies the logging as it is now possible
to enable different level of debugging output for each network
module individually.

The commit also converts the code to use the new logger
instead of the old sys_log.

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