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: 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.