restrict http send buffer to 4096

send() is able to complete but not send everything asked...
we don't deal with that yet.  On an ARM9 board, leaf.jpg
failed because the send() only accepted 7240 of 8192... not
a failure but it implies reporting partial send to the
caller.  Work around by keeping the send buffer at 4096.

Signed-off-by: Andy Green <andy.green@linaro.org>