subsys: console: Refactor code to allow per-UART "tty" wrapper

Instead of having just one static "UART console" device, allow to
instantiate buffered, interrupt-driven POSIX-like "tty" wrapper for
any underlying device. Then, use this functionality to provide
compatible "console API".

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