The dhcp command is supposed to have the following syntax as per
"help dhcp":
dhcp [loadAddress] [[hostIPaddr:]bootfilename]
In other words, any arguments should be passed to an implicit
tftpboot command after the DHCP exchange has occurred.
Add the missing code to the lwIP version of do_dhcp().
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>