If we are booting pxe booting using syslinux, and it has IPAPPEND 2 in
the boot stanza, then it will append the mac address of the device we
are booting from to the kernel parameters where we can get at it
pretty easily.
If we are booting physical hardware via UEFI over the network, we can
rely on the BootCurrent EFI variable to point at the boot entry for
the NIC we booted from, which will include the MAC address of that
nic.
If neither of those cases are in play, we can just fall back to the
all-physical-devices-with-links code.
This currently uses the Bash 4 support for associative arrays to handle
the netboot-in-UEFI case, if needed I can rewrite it to be Bash 3 compatible.
Change-Id: I5e50e30c60d6d732a09ab61251cbb9be08bb6113
dbacf3e8df