TFTP: allow for adjustable retransmission timout

So far, TFTP negotiated a fixed retransmission timeout of 5 seconds.
In some cases (busy networks, slow TFTP servers) this caused very
slow transfers. A new environment variable "tftptimeout" allows to
set this timeout. Lowering this value may make downloads succeed
faster in networks with high packet loss rates or with unreliable
TFTP servers.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Ben Warren <biggerbadderben@gmail.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>