cmd: Kconfig: Fix submenu for network commands

cmd: Kconfig: Fix submenu for network commands

The Kconfig parser seems to get confused by the current if conditions
following CMD_NET and displays all network command options directly in
the "Command line interface" menu instead of in a "Network commands"
submenu.

To help out Kconfig we can simplify the if conditions, so that the
definition of CMD_NET is followed immediately by an if/endif block that
contains all network command options. We can also remove nested checks
for CMD_NET or (NET || NET_LWIP).

Fixes: 98ad145db61a ("net: lwip: add DHCP support and dhcp commmand")
Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>