This adds tftp filesystem support. It currently duplicates
significant amounts of the tftp (command) support. This is ok
since we can eventually drop the original tftp command later.

tftp is not really suitable to be handled as a filesystem. It lacks
support for stat, reading directories and other things. Handling
it as a filesystem has one big advantage though: tftp is no special
case for boot scripts and/or commands anymore which makes them simpler.

This implementation has some improvements to the original tftp command.
It supports blocksize negotiation which speeds up transfers if the tftp
server supports it. Also we can determine the filesize to transfer if
the remote end supports it.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>