Convert license tags to SPDX

Also clarifies provenance of code in `apps/fdtgen/fdt_utils.c`

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

picotcp_tcp_echo: Add utilization and tracing

- Add a socket for starting and stopping and reporting utilization
information gathering
- Add BenchUtiliz component for performing measurements and also tracing
support.

picotcp_tcp_echo: Use async socket interfaces

These async interfaces use virtqueues to batch multiple receive and send
operations together. This change applies to both the TCP connections and
UDP socket. The socket functionality was moved into separate files also.

picotcp_tcp_echo: Create app based on picoserver

This app is similar to picoserver but doesn't have a listener component.
It has an echo client that echoes TCP messages received on port 1234