net: dhcp: rework ...

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>

net: dhcp: Allow to specify network device ...

Instead of allowing to DHCP only on the "current" network
device, allow to specify the desired network device. This
is a first step to get rid of the concept of a "current"
network device.

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

commands: exit on invalid option ...

Barebox commands should not perform any action and return 0 if an
invalid parameter was given. This might cause undetected unintended
behvaior when calling commands with wrong options, either manually or
from a script.

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

net: dhcp: Split dhcp funcionality & add dhcp command ...

dhcp/bootp was bound to the command functionality.
This patch splits the dhcp command.
We are now able to use bootp without a shell.

This patch adds also a check for environment variables and globalvar.
So only when ENVIRONMENT_VARIABLES and GLOBALVAR is set, all received
dhcp data will be stored.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>