discover: extend URL in UI to support auto-discovery

The URL field currently only supports loading a particular file for
static network configurations. But it makes sense in certain static
network configurations to 'auto-discover' a file like petitboot does
with DHCP -- based off the MAC address and IP. Extend
device_handler_process_url to take those as parameters, and toggle off
the URL ending in a '/' to indicate whether to 'auto-discover' or
directly load the specified URL.

Signed-off-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
Signed-off-by: Samuel Mendoza-Jonas <sam.mj@au1.ibm.com>