eth-rmii-test: support xsim as well as verilator
sync_oneway: add timescale directive
axi-sram: handle axi3 lengths (4 vs 8bit) and single-count bursts
eth-rmii-rx: start of packet strobe

useful for timestamping
eth-capture and friends: capture inbound frames to axi

axi_dma_writer: commit 1-16 word bursts over axi
pkt_bytes_to_words: convert bytestream to wordstream
eth_capture: capture packets to a 4MB ring (currently at 0x10000000)

- packets are located at 4k offsets
- first 4 words of each packet are
  - bytecount
  - timestamp(low)
  - timestamp(high)
  - status(1=packet written)
- data starts at 64 bytes in to each packet
eth-capture-test basic packet capture testbench