net: Provide a function to run network operations

net: Provide a function to run network operations

Add a new netboot_run() function which can be used for simple network
operations, such as loading a file. Put the implementation in an
internal function, used by the existing code.

Place this function into the net/ code, so that it does not need the
command line to be available.

Document which network operations are supported, i.e. a limited subset,
for now.

For the one board which uses lwip, it is not quite clear how to avoid
using the cmdline interface. This will need some discussion.

Signed-off-by: Simon Glass <sjg@chromium.org>