lib_xtcp general updates and enhancements

[humphrey-xmos]

A few updates that would improve the library,

• Swap lib_xtcp LwIP API to LWIP_ALTCP, would be needed for TLS integration.
• Timestamps: Current API only builds with real-time MACs. Create config to exclude timestamp API calls and handling. To allow non-rt MAC.
• Client queue: Refactor client queue.
• Implement i_xtcp.abort(), currently uses close() only,
• Split shim_send() for UDP and TCP send()
  • In send, move freeing of pbufs to send interface function. As allocation is done via calls to pbuf_shim_alloc_tx()and the call to shim_send() is assumed to free the buffer. A bit asymetric.
• Look at linking lib_ethernet conf headers to lib_xtcp conf headers. To simplify config changes.
• Add link-speed value to up/down notification.
• Create low resource example (minimal lwipopts.h already in library)
• Should recv() return XTCP_ENOMEM when no pbuf available? Currently returns XTCP_EINVAL

Testing

• TEST: UDP equivalent to flood test
• Create XCORE to PC connection tests. projects already exist.