The following condition has been confirmed on 2 NUC units as follows:
DC3217IYE
Bios v. 0055 release: 12/5/14
When network booting is enabled and the boot order is arranged so that the LAN network boot (pxe) is the first source, this setting is not honored.
After "exit & save" and a subsequent restart the hard drive (in this case SSD) remains the first boot device.
Thereafter reentering BIOS shows the LAN to be a subordinate to the on-board drive.
The only method to LAN boot at this time is to enter BIOS and use the "double click" on the listed LAN boot option on the main page.
This condition did not exist on BIOS versions prior to 0055.
I would also add that it is long overdue for INTEL to directly support flashing a BIOS pain-free with Linux.