Jenkinsfile: build against analogous manifest branch

Currently, every pull request and branch build is built against master
branch of the pelux-manifests, which is incorrect since both meta layers
and pelux-manifests follow the Yocto branching strategy, making some of
the branches incompatible between each other.

Build the layer against the corresponding branch of the PELUX manifest
instead, e.g. sumo against sumo and master against master.

Signed-off-by: Oleksandr Kravchuk <oleksandr.kravchuk@pelagicore.com>

scripts/Jenkinsfile

@@ -1,17 +1,27 @@
 #!/usr/bin/groovy
 
-// Copyright (C) Luxoft Sweden AB 2018
+// Copyright (C) 2018 Luxoft Sweden AB
 
 void buildBistro(String variant, String imageName) {
     String bistroDir = "meta-bistro"
+    String branchName = "master"
+
+    // If this is a PR, CHANGE_TARGET will be set by the Pipeline Plugin,
+    // otherwise check BRANCH_NAME which is set by Jenkins
+    if (env.CHANGE_TARGET) {
+        branchName = "${env.CHANGE_TARGET}"
+    } else if (env.BRANCH_NAME) {
+        branchName = "${env.BRANCH_NAME}"
+    }
+
     node("Yocto") {
         dir(bistroDir) {
             checkout scm
         }
 
         // Initialize pelux-manifests to get the code
         sh "rm -rf pelux-manifests/"
-        sh "git clone https://github.com/Pelagicore/pelux-manifests.git"
+        sh "git clone https://github.com/Pelagicore/pelux-manifests.git -b ${branchName}"
 
         dir('pelux-manifests') {
             def code = load "ci-scripts/yocto.groovy"
@@ -32,4 +42,3 @@ variantList.each {
 }
 
 parallel (variantMap)
-