lwip: split cmd/net-lwip.c into one file per command

Move each command in cmd/net-lwip.c into its own file
(cmd/lwip/${cmd}.c).

Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
8 files changed