uart_native_tty: Emulate an interrupt driven uart

Emulate SERIAL_SUPPORT_INTERRUPT for UART_NATIVE_TTY, using a thread that
polls the tty and invokes the callback.

This allows interrupt-driven subsystems such as modbus to use a native tty,
which is useful for testing purposes.

Signed-off-by: Björn Stenberg <bjorn@haxx.se>