This change decouples the IEEE 802.15.4 (L2) layer from all IPv6 (L3)
concerns.
Applications may now choose to set CONFIG_NET_6LO=n and
CONFIG_NET_L2_IEEE802154=y at the same time.
Setting CONFIG_NET_6LO=n will build a vanilla IEEE 802.15.4-2006 specs
compliant L2 layer without any reference to 6LoWPAN or IPv6. This allows
application developers to design custom non-IP protocols on top of
IEEE 802.15.4-2006 and thereby makes the L2 layer much more re-usable.
Fixes#48585.
Signed-off-by: Florian Grandel <jerico.dev@gmail.com>