Nokia RX-51: Add automated test for running RX-51 build in qemu

Nokia RX-51: Add automated test for running RX-51 build in qemu

This patch contains test/nokia_rx51_test.sh script which automatically
download and compile all needed tools in local temporary directory to
generate a simple MTD images for booting Maemo kernel image by U-Boot from
RAM, eMMC and OneNAND. MTD images are then run in virtual n900 machine
provided by qemu-linaro project.

This script does not need any special privileges, so it can be run as
non-root nobody user.

It can be used to check that U-Boot for Nokia N900 is not broken and can be
successfully booted in emulator.

Script is registered to .azure-pipelines.yml, .gitlab-ci.yml and
.travis.yml so it would be automatically run on those CI services.

Signed-off-by: Pali Rohár <pali@kernel.org>