net: Move the DHCP command below the BOOTP command

Move DHCP to directly follow BOOTP so that Kconfig can show the
dependency as a hierarchy.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>

Reviewed-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Duncan Hare <dh@synoia.com>