[gPXE] fall-back filename

[Miller, Shao]

Good day Alex,

In regards to your challenge of testing for a populated gPXE setting:

Please try:

set taken:hex ${filename:hex}:00

instead of '${take} ${filename}'.  Feel free to let me know the results.

Boot.Kernel.Org needed something similar; to test for IP settings being
obtained via DHCP.  I think this is what was landed on for that case.

- Shao Miller