[gPXE] clear filename?

[Miller, Shao]

Good day Arintel,

If it's ProxyDHCP, it'll be in yet another scope.  You could try
clearing them all, just for fun:

clear filename
clear net0/filename
clear net0.dhcp/filename
clear proxydhcp/filename
echo Left-over filename is "${filename}"

The 'echo' line should say:

Left-over filename is ""

If it does, you are re-populating the filename option at some point
after that.

- Shao Miller