subsys: console: tty: Allow to specify transmit timeout

Previously, transmit was effectively non-blocking - a character either
went into buffer, or -1 was returned. Now it's possible to block if
buffer is full. Timeout is K_FOREVER by default, can be adjusted
with tty_set_tx_timeout() (similar to receive timeout).

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>