net: shell: NET_SHELL is not NET_LOG dependent

And it's not really a debugging option either, one might want to use it
for the features it brings. Thus moving it out of Kconfig.debug.

Change-Id: Ie1e2d2bcba94ad4b8354d3cc56087227184039b6
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>