Use the 418-devkit branch for legacy images.

Signed-off-by: Angus Ainslie (Purism) <angus@akkea.ca>

Jenkinsfile

 node {
     gitUrl = 'https://source.puri.sm/librem5/u-boot-builder.git'
     artifacts = 'output/uboot-*/*'
-    branch = 'master'
+    branch = '418-devkit'
 
     parameters {
         string(name: 'board', defaultValue: "devkit", description: 'What board type to build for?')