Change the behaviour for adding DHCP options into a DHCP packet so
that we now append options, rather than insert them in front of
whatever options might already be present.

Apparently, the DHCP relay logic on a Nortel 470-48T layer 2 switch
cares about the order of DHCP options.  If we build a DHCP packet
pre-populated with some options, their order will now be preserved,
except for encapsulated options.

Signed-off-by: Marty Connor <mdc@etherboot.org>