net: Add a function to run dhcp

At present this must be done by executing the command. Also it involves
fiddling with the environment to determine the correct autoload behaviour.

Ideally it should be possible to run network operations without even
having the command line present (CONFIG_CMDLINE).

For now, add a function to handle DHCP, so it can be called from a bootdev
more easily.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>