Nokia RX-51: Add test for U-Boot serial console

This patch adds a new test which checks that U-Boot for Nokia RX-51 running
in qemu can print test line to serial console and also checks that test
line appeared on qemu serial console.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Pavel Machek <pavel@ucw.cz>