ci: Add fvpboot to IMAGE_CLASSES

Use IMAGE_CLASSES rather than a direct INHERIT for fvpboot.
This is Yocto best practice as it is used to enable
functionality across all image recipes.

Signed-off-by: Debbie Martin <Debbie.Martin@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

ci/fvp.yml

@@ -4,7 +4,7 @@ header:
 local_conf_header:
   testimagefvp: |
     LICENSE_FLAGS_ACCEPTED += "Arm-FVP-EULA"
-    INHERIT += "fvpboot"
+    IMAGE_CLASSES += "fvpboot"
   failing_tests: |
     # This fails but we can't add to the ignorelist from meta-arm yet
     # https://bugzilla.yoctoproject.org/show_bug.cgi?id=14604