ci: Enable ccache for buildroot

I wanted to use a persistent volume for the workspace volume of this
Jenkins pipeline in order to speed up subsequent builds.  Unfortunately,
this feature does not really work well in Jenkins.  Since the agent is
removed when the build completes, and therefore the volume is not
mounted anywhere, Jenkins cannot delete workspaces for deleted jobs.
This means that when feature branches/pull requests are merged, their
workspaces remain on disk forever.

Using the *dynamicPVC* option deletes the entire volume when the job
finishes.  This avoids wasting space, but does not allow subsequent
builds to reuse the workspace.  As a compromise, we can use a persistent
volume for the compiler cache and share it between builds.  Using the
compiler cache is not as effective as keeping the whole build directory,
but it still saves some time.  Also, it works across branches, too.

ci/Jenkinsfile

@@ -5,9 +5,7 @@ pipeline {
         kubernetes {
             yamlFile 'ci/podTemplate.yaml'
             defaultContainer 'buildroot'
-            workspaceVolume persistentVolumeClaimWorkspaceVolume(
-                claimName: 'jenkins-ws-pythonctnr'
-            )
+            workspaceVolume dynamicPVC()
         }
     }
 
@@ -16,20 +14,9 @@ pipeline {
         disableConcurrentBuilds()
     }
 
-    parameters {
-        booleanParam \
-            name: 'Clean',
-            description: 'Clean the workspace and perform a full rebuild'
-    }
-
     stages {
         stage('Prepare') {
             steps {
-                script {
-                    if (params.Clean) {
-                        sh 'rm -rf _build'
-                    }
-                }
                 checkout poll: false, scm: [
                     $class: 'GitSCM',
                     branches: [[name: '2022.05.x']],

ci/podTemplate.yaml

@@ -7,9 +7,14 @@ spec:
     command:
     - sleep
     - infinity
+    env:
+    - name: HOME
+      value: /home/jenkins
     volumeMounts:
     - name: tmp
       mountPath: /tmp
+    - name: br2-ccache
+      mountPath: /home/jenkins/.buildroot-ccache
     securityContext:
       readOnlyRootFilesystem: true
       runAsUser: 1000
@@ -25,3 +30,6 @@ spec:
     emptyDir:
       medium: Memory
       sizeLimit: 100Mi
+  - name: br2-ccache
+    persistentVolumeClaim:
+      claimName: jenkins-br2-ccache

ci/pvc.yaml

@@ -5,10 +5,10 @@
 apiVersion: v1
 kind: PersistentVolumeClaim
 metadata:
-  name: jenkins-ws-pythonctnr
+  name: jenkins-br2-ccache
 spec:
   accessModes:
   - ReadWriteOnce
   resources:
     requests:
-      storage: 10Gi
+      storage: 20Gi

configs/pythonctnr_defconfig

@@ -1,4 +1,5 @@
 BR2_x86_64=y
+BR2_CCACHE=y
 BR2_TOOLCHAIN_BUILDROOT_GLIBC=y
 BR2_TARGET_GENERIC_HOSTNAME="pythonctnr"
 BR2_TARGET_GENERIC_ISSUE=""