CI: clean up variable definitions

Mostly taken from master, rationalise the variable definitions and stop
hardcoding /persist/.

Users of this CI will have to ensure that they've set CACHE_DIR if they
want anything actually cached.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

.gitlab-ci.yml

@@ -7,6 +7,10 @@ variables:
   # by default
   FF_USE_LEGACY_KUBERNETES_EXECUTION_STRATEGY: 0
   FF_KUBERNETES_HONOR_ENTRYPOINT: 1
+  # The directory to use as the persistent cache (the root for DL_DIR,
+  # SSTATE_DIR, etc). The default is the build tree which will not be
+  # persistent, so this should be set in the runner.
+  CACHE_DIR: $CI_PROJECT_DIR
 
 stages:
   - prep
@@ -20,11 +24,12 @@ stages:
   interruptible: true
   variables:
     KAS_WORK_DIR: $CI_PROJECT_DIR/work
-    KAS_REPO_REF_DIR: $CI_BUILDS_DIR/persist/repos
-    SSTATE_DIR: $CI_BUILDS_DIR/persist/sstate
-    DL_DIR: $CI_BUILDS_DIR/persist/downloads
+    KAS_BUILD_DIR: $KAS_WORK_DIR/build
+    KAS_REPO_REF_DIR: ""
+    SSTATE_DIR: $CACHE_DIR/sstate
+    DL_DIR: $CACHE_DIR/downloads
     BB_LOGCONFIG: $CI_PROJECT_DIR/ci/logging.yml
-    TOOLCHAIN_DIR: $CI_BUILDS_DIR/persist/toolchains
+    TOOLCHAIN_DIR: $CACHE_DIR/toolchains
     IMAGE_DIR: $CI_PROJECT_DIR/work/build/tmp/deploy/images
     TOOLCHAIN_LINK_DIR: $CI_PROJECT_DIR/work/build/toolchains
   before_script: