discover/pxe: Format IPAPPEND mac addresses correctly
The SYSAPPEND/IPAPPEND option 2 in PXE configs requires
the MAC address of the booting interface to be appended
to the boot options. Previously we formatted this as
"BOOTIF=01:02:03:04:05:06",
but syslinux/pxelinux implementation use this format:
"BOOTIF=01-01-02-03-04-05-06",
where the leading '01' represents the hardware type.