The DHCP code is a mess. It is not clear which options are sent to the
server and which options are returned from the server. Also environment
variables are read from and written to all over the place.

This patch cleans this up. There now is struct dhcp_req_param which is
used for options sent to the server and struct dhcp_result which contains
the values sent from the server. The values from the server are written
to the barebox variables in a single place. Also it's now possible to
call the dhcp code without modifying barebox variables at all, storing
the result only in the dhcp result struct.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>