Add Transfer Size Option to tftp

Optionally add RFC 2349 "Transfer Size Option", so we can minimize the
time spent sending data over the UART (now print a single line during a
tftp transfer).

 - If turned on (CONFIG_TFTP_TSIZE), U-Boot asks for the size of the file.
 - if receives the file size, a single line (50 chars) are printed.
     one hash mark == 2% of the file downloaded.
 - if it doesn't receive the file size (the server doesn't support RFC
     2349, prints standard hash marks (one mark for each UDP frame).

Signed-off-by: Robin Getz <rgetz@blackfin.uclinux.org>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>