Since FOG seems to be using SANBOOT for exiting is your HP Server in BIOS (legacy) mode or UEFI. The answer is different based on the mode the server firmware is in.
For this server, go into the host definition. Lets assume this sever is in bios mode, change the BIOS exit type to GRUB First Hard Drive. Hint: You may need to experiment with the ext types to find the right one that works with your hardware. The rEFInd exit type is for UEFI hardware.
The other thing I can say (at least for my company) we don’t have servers pxe boot through FOG. When we go to image a physical machine we press the F10 or F12 to bring up the boot manager during the system POST. Then from the boot manager we select PXE boot into FOG. This way we are sure there is no way to accidentally reimage a server. We have a rule that a technician must be in front of the server before it is allowed to be reimaged. If you use this method the hard drive is always defined first in the boot order.