The "dhcp" command now accepts a list of interfaces to try until one
succeeds.  For example:

 iPXE> dhcp net0 net1 net2

If no interfaces are specified, all interfaces will be tried.

Note that interfaces that fail to DHCP are closed in order to avoid
memory exhaustion.  This behavior differs from the previous "dhcp"
command implementation but should not affect any existing scripts
since a "dhcp" command failure would in any case cause the script to
abort.

Originally-implemented-by: Lars Kellogg-Stedman <lars@oddbit.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>