net/dhcp: Add explicit net_dhcp command

net/dhcp: Add explicit net_dhcp command

Mostly for cosmetic reasons, we add a "net_dhcp" command, which is (at the
moment) identical to the existing "net_bootp" command. Both actually trigger
a DHCP handshake now, and both should be able to deal with pure BOOTP servers.
We could think about dropping the DHCP options from the initial DISCOVER packet
when the user issues the net_bootp command, but it's unclear whether this is
really useful, as both protocols should be able to coexist.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>