CI: travis: Add boot test for i.MX8X

i.MX8X is now supported in qemu up to boot. So, add
it to boottest list.

Signed-off-by: Diana Cretu <diana.cretu@nxp.com>

Author: Diana Cretu

.travis.yml

@@ -70,3 +70,5 @@ jobs:
       env: PLATFORM=icl
     - <<: *qemuboottest
       env: PLATFORM=imx8
+    - <<: *qemuboottest
+      env: PLATFORM=imx8x