net: offload: Rename Kconfig NET_L2_OFFLOAD_IP to NET_OFFLOAD

This is part of a patch series to decouple TCP/IP offload, which
currently occurs at the transport layer (L4), from the data link
layer (L2).

This patch simply renames the NET_L2_OFFLOAD_IP Kconfig variable
to NET_OFFLOAD.

Change-Id: Ic8b1d004cbac09b7c636475aaed75b0a31e4be1c
Signed-off-by: Gil Pitney <gil.pitney@linaro.org>