CI: add task to delete all sstate

Change-Id: If89c6c27cafe6ae7e93b5db5d37b9e8ecdaee131
Signed-off-by: Ross Burton <ross.burton@arm.com>

.gitlab-ci.yml

@@ -99,3 +99,11 @@ prune-sstate:
   when: manual
   script:
   - find $SSTATE_DIR -type f -atime +30 -delete
+
+# Delete all sstate
+delete-sstate:
+  extends: .setup
+  stage: bootstrap
+  when: manual
+  script:
+  - rm -rf $SSTATE_DIR