/usr/lib/ipxe/pcnet32.rom is in ipxe-qemu 1.0.0+git-20180124.fbe8c52d-0ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/ipxe/qemu/pxe-pcnet.rom
/usr/lib/ipxe/qemu/pxe-pcnet.rom