Slave: Update TF-M jobs to use Bionic over Xenial

Signed-off-by: Karl Zhang <karl.zhang@arm.com>
Change-Id: Icf49df65bc80697b5e02aef1d18e863ff5187127
diff --git a/jenkins/cppcheck.jpl b/jenkins/cppcheck.jpl
index 34e23b1..1aad8bd 100644
--- a/jenkins/cppcheck.jpl
+++ b/jenkins/cppcheck.jpl
@@ -9,7 +9,7 @@
 @Library('trustedfirmware') _
 import org.trustedfirmware.Gerrit
 
-node("docker-amd64-xenial") {
+node("docker-amd64-bionic") {
   stage("Init") {
     cleanWs()
     dir("trusted-firmware-m") {