The eboot loader does not support the Broadcom NetXtreme 57xx NIC.
The default eboot loader support NE2000 compatible NIC, Realtek-8139 and a few other.
To get the Broadcom NIC to work involve some major driver codes.
You can try hunting for one of the legacy PCMCIA NIC that is NE2000 compatible.
You can post your question to the following forum, which will reach a much broader Windows CE developer.