net: split net into net{,-common,-legacy,-lwip}

Make net.h a wrapper which includes net-common.h and either
net-legacy.h or net-lwip.h based on NET_LWIP. The function
copy_filename() can be useful when NET_LWIP is enabled, therefore
move it out of net/net.c which is built only when networking choice
is NET and create a new file net/net-common.c.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>