net: socket: Make the send timeout configurable

When the protocol layer like TCP is blocking transmission, the socket
layer will attempt and wait for a maximum amount of time before returning
with an ENOBUFS error.
This change allows to set the maximum waiting time from the configuration
file instead of using a fixed 10 second value.

Signed-off-by: Sjors Hettinga <s.a.hettinga@gmail.com>