Support adding DHCP interfaces one at a time.

Refactors dhcp-all-interfaces.sh so that if an optional
INTERFACE argument (the first argument) is passed to the script
it only inspects that single interface. If no argument is
passed then the previous default behaviour use used which
causes all interfaces to be inspected.

Also sets PATH so that the commands within the script
can all be found if it isn't set properly (/sbin/ip, /bin/cat, etc.)

This is a move towards using udev rules to add these types
of interfaces automatically.

Change-Id: Ia482c1d3ddce0f0d8d77f9bc3ac76d6924640715
Merge "Support adding DHCP interfaces one at a time."