Since spike doesn’t implement a virtual Ethernet device, eth0 being absent is expected. Keep in mind that the SDK is originally intended for use with the VC707 development board to which a physical NIC can be connected via PCIe. It should be possible to modify the buildroot to exclude the networking initscript.
At this time, spike doesn’t support external block devices, so there is no way to mount additional filesystems. You will have to add whatever files you need to the buildroot initramfs using a filesystem overlay. Unfortunately, this means that file modifications aren’t persistent.