CI: Add BUILD_ENABLE_REGEX option to conditionally enable builds

Mirrors of meta-arm may focus their development on a small subset of
MACHINEs so provide the option to restrict the boards that are built on
CI using the variable BUILD_ENABLE_REGEX. If set, it conditionally
enables builds; if unset there is no change in behavior.

This variable could be overridden in a scheduled build, to e.g. build
all the MACHINEs weekly.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

.gitlab-ci.yml

@@ -42,6 +42,9 @@ stages:
   extends: .setup
   variables:
     KUBERNETES_CPU_REQUEST: $CPU_REQUEST
+  only:
+    variables:
+      - $BUILD_ENABLE_REGEX == null || $CI_JOB_NAME =~ $BUILD_ENABLE_REGEX
   script:
     - KASFILES=$(./ci/jobs-to-kas "$CI_JOB_NAME")
     - kas dump --update --force-checkout --resolve-refs --resolve-env $KASFILES