ci: Add CUSTOM_TARGET parameter

If a value is provided for the `CUSTOM_TARGET` parameter, the specified
target will be built first, before building the filesystem images.  This
provides a way to e.g. rebuild a specific package.

ci/Jenkinsfile

@@ -1,6 +1,7 @@
 pipeline {
     parameters {
         booleanParam 'CLEAN_BUILD'
+        string 'CUSTOM_TARGET'
     }
 
     options {
@@ -45,6 +46,9 @@ pipeline {
         stage('Build') {
             steps {
                 sh 'make -C buildroot O="${PWD}"/_build BR2_EXTERNAL="${PWD}" aimeeos_qemu_aarch64_defconfig'
+                if (params.CUSTOM_TARGET) {
+                    sh "make -C _build '${CUSTOM_TARGET}'"
+                }
                 sh 'make -C _build'
             }
             post {