net/6lo: Enable it by default if IEEE 802.15.4 is enabled

This rework commit 77b8f5c1

Comparing it to BT IPSP is a the wrong comparison: BT IPSP does specify
6lo/ipv6 for it to work. Whereas 802.15.4 does not.

Instead of selecting 6lo from 802.15.4's Kconfig, let's do the reverse
way. If the user enabled 802.15.4 and IPv6 as well (to which 6lo
depends on), then 6lo is enabled by default as using IPv6 on 15.4
without it does not make much sense.

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