iPXE: Fix assumption that ${mac} is the MAC of the NIC it's booting
iPXE: Fix assumption that ${mac} is the MAC of the NIC it's booting

The ${mac} variable not necessarily points to the MAC address of the NIC
we are booting from. This patch updates the boot.ipxe script to make it
more robust by interating over all NICs found in the machine and boot
from the first one capable of booting (the one that contains a
configuration file in the server). If no NICs are capable of booting the
script will fail just like before telling the user that the network boot
failed because we couldn't find any configuration for the present NICs.

Closes-Bug: #1504482
Change-Id: I5b8f0740fa42f611e84b66dc7f6578cffe02760a

dims/ironic,ionutbalutoiu/ironic,hpproliant/ironic,NaohiroTamura/ironic,ionutbalutoiu/ironic,pshchelo/ironic,NaohiroTamura/ironic,openstack/ironic,openstack/ironic,bacaldwell/ironic,devananda/ironic,SauloAislan/ironic,SauloAislan/ironic,pshchelo/ironic,dims/ironic,bacaldwell/ironic