feat: Draft parallel execution

Jenkinsfile

@@ -56,14 +56,15 @@ pipeline {
             def targets_string = sh(script: 'make list-targets', returnStdout: true)
             def targets = targets_string.tokenize('\n')
             def targets = ['ath79-generic','ipq40xx-generic'] // override for testing purposes
-            targets.each { target_name ->
+            def build_stages = {targets.each { target_name ->
               stage("Build ${target_name}") {
                 echo "${target_name}"
-                def built = build(job: "nightly-wireguard", wait: false, propagate: false, parameters: [
+                def built = build(job: "nightly-wireguard", wait: true, propagate: false, parameters: [
                   string(name: 'GLUON_TARGET', value: "${target_name}")
                 ])
               }
-            }
+            }}
+            parallel build_stages
           }
         }
       }