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[NiKiZe]

I would not replace any of them.
You should build a rom for your nic, and then refer to that.
Assuming you are using legacy pcbios and not efi ...
If you are using rtl8139, then build with:
make bin/10ec8139.rom

Why are you trying to build for the broadcom nic?
guessing here, correct me if I'm wrong... you want this on real hw for chainbooting?
For real hardware, use eighter undionly.kpxe or ipxe.pxe (assuming pcbios again here)
The reason for referring to the alternate rom is only to have your 2 nics to work inside of qemu - not relevant to other hardware.

With that said - to detect iPXE roms and if it needs to chain into a "fuller" iPXE you can use something like: https://gist.github.com/robinsmidsrod/4008017