CI: add cache for builds

Build cache is kept per build target. Each tag also has a seperate build
cache, so tagged builds are always built from scratch.

.gitlab-ci.yml

@@ -24,6 +24,9 @@ stages:
     paths:
       - gluon/output/
     expire_in: 1 day
+  cache:
+    untracked: true
+    key: "$CI_COMMIT_TAG-$GLUON_TARGET"
  
 target:ar71xx-generic:
   extends: .target