net: ip: Improve logging by adding a dedicated sys_log level

Let's make net stack having its own level of debugging through sys_log.
It replaces NET_DEBUG by NET_LOG_ENABLED, which is then semantically
better: someone wanting to log the errors might want that not only for
debugging.

Along with it, CONFIG_NET_LOG_GLOBAL option is added, in order to enable
all available logging in network stack. It is disabled by default but
might be found useful when warning/errors need to be logged, so it is
then unnecessary to selectively enable by hand all CONFIG_NET_DEBUG_*
options.

It is possible, locally, to override CONFIG_SYS_LOG_NET_LEVEL by setting
the level one want to NET_SYS_LOG_LEVEL. This can be useful on samples
or tests.

Change-Id: I56a8f052340bc3a932229963cc69b39912093b88
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>