[netdevice] Add generic concept of a network device configurator

[netdevice] Add generic concept of a network device configurator

iPXE supports multiple mechanisms for network device configuration:
DHCPv4 for IPv4, FIP for FCoE, and SLAAC for IPv6.  At present, DHCPv4
requires an explicit action (e.g. a "dhcp" command), FIP is initiated
implicitly upon opening a network device, and SLAAC takes place
whenever a RA happens to be received.

Add a generic concept of a network device configurator, which provides
a common interface to triggering configuration and to reporting the
result of the configuration process.

Signed-off-by: Michael Brown <mcb30@ipxe.org>