Allow a single DHCP configuration via the system configuration submenu

Allow a single DHCP configuration via the system configuration submenu

This patch allows the setup of simple a single interface to be
automatically brought up and configured via DHCP on system startup.

The interface name can be set via a configuration option. This patch
does not support systemd-networkd, any complex network configuration should
be done via overlay of /etc/network/interfaces or the relevant networkd
configuration file

[Peter: rename to BR2_SYSTEM_DHCP, tweak help text & implementation]
Signed-off-by: Jérémy Rosen <jeremy.rosen@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>