Revert Beaglebone test image to 2018-08-30 revision.

This image is old enough to have a U-Boot version from before the boot
problems in MEN-2404 were introduced (>= v2018.09). At the time of
writing, no image has been released which has a U-Boot from after the
problem was fixed (> v2019.07), but we expect it will eventually be
released, and then we can bump to that version.

Changelog: None

Signed-off-by: Kristian Amlie <kristian.amlie@northern.tech>

scripts/test/run-tests.sh

@@ -15,7 +15,7 @@ fi
 
 WORKSPACE=./tests
 
-BBB_DEBIAN_IMAGE="bone-debian-9.5-iot-armhf-2018-10-07-4gb.img"
+BBB_DEBIAN_IMAGE="bone-debian-9.5-iot-armhf-2018-08-30-4gb.img"
 BBB_DEBIAN_IMAGE_URL="http://debian.beagleboard.org/images/${BBB_DEBIAN_IMAGE}.xz"
 
 RASPBIAN_IMAGE="2019-09-26-raspbian-buster-lite"