net: app: Fix debug level check

We need to check the debug level to be INFO or higher, otherwise
the some of the variables in net app init.c will give unused
warning. Also the sys_log.h needs to be included _after_ we have
set the logging level, this is done by net_core.h so we should
not include the syslog header file here.

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