tests/functional: Add a functional test for the sx1 board

tests/functional: Add a functional test for the sx1 board

Add a functional test for the sx1 board that uses the kernel and
rootfs provided by Guenter Roeck in the linux-test-downloads repo:
 https://github.com/groeck/linux-test-downloads/

We have three variants of the test for this board:
  * just boot initrd
  * boot with filesystem on SD card
  * boot from flash

In all cases these images have a userspace that is configured to
immediately reboot the system on successful boot, and the board
itself supports telling QEMU to do the reboot, so we only need to
wait for QEMU to exit (via -no-reboot).

Since there are three subtests, the test as a whole takes about
80s on my local machine. That's about the same as the aarch64_virt
test, so give it the same overall test timeout as that one.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20241017163247.711244-3-peter.maydell@linaro.org