net: split cmd/net.c into cmd/net.c and cmd/net-common.c

Extract some code from cmd/net.c that will be useful in a subsequent
commit to implement wget with NET_LWIP.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
3 files changed