uboot/kernel: Only send mail on failed builds

As I learned today mails from Jenkins were so far held by list moderation. Now that they flow in it'd be nice to only have them when a build fails. E.g. like:

https://stackoverflow.com/questions/39720225/send-email-on-jenkins-pipeline-failure

@angus.ainslie since this affects kernel and uboot could you have a look (since I'm on my way back from GUADEC)