Add MCUBoot dependency for doc building

Change-Id: I820257e3104d3465ffe71ed38df6ee5f0e80c5c1
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
diff --git a/jenkins/build-docs.jpl b/jenkins/build-docs.jpl
index c759fb1..e434090 100644
--- a/jenkins/build-docs.jpl
+++ b/jenkins/build-docs.jpl
@@ -42,6 +42,20 @@
         ]
       )
     }
+    dir("mcuboot") {
+      checkout(
+        changelog: false,
+        poll: false,
+        scm: [
+          $class: 'GitSCM',
+          branches: [[name: 'FETCH_HEAD']],
+          userRemoteConfigs: [[
+            refspec: 'refs/tags/$MCUBOOT_VERSION',
+            url: params.MCUBOOT_URL
+          ]]
+        ]
+      )
+    }
     sh """
     # Host https://github.com/Arm-software/CMSIS_5/releases/download/5.5.0/ARM.CMSIS.5.5.0.pack
     # under \$JENKINS_HOME/userContent