subsys/net/ip: Clear up content in CMakeLists.txt

Empty CMakeLists.txt in l2 and l2/ieee802154 deserved to get filled-in
relevantly, instead of centralizing everything in ip/ location.
Also making sure lines don't get over 80 chars.
Also, no need of linking against mbetls unless net shell is enabled.

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