subsys/console: Yield on char availability.

If this is not done, there is common pattern that on big input block
(e.g. from a clipboard paste), IRQ routine is called in a tight loop,
leading to circular buffer overflow.

Change-Id: I69a7aa78081b8d74652406f3b3a577ddaf4c5f6f
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>