Allow images to declare dependencies

A new parameter, 'requires', allows declaring a dependency on
a different image build.

Instead of a single artifact list, define two lists.
First build artifacts not requiring any other artifacts,
then build artifacts depending on other artifacts.

This code doesn’t support transitive dependencies.

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>

Jenkinsfile

@@ -57,6 +57,9 @@ def architectures = [
                 image: true,
                 sysroot: false,
                 ostree: false,
+                requires: [
+                    armhf: 'development',
+                ]
             ]
         ]
     ],
@@ -101,6 +104,7 @@ def architectures = [
         ]
     ]
 ]
+
 // SSH complains loudly if the current user is not in the user database, so fake that
 def withSshAgent(credentials, command) {
   sshagent(credentials: [ credentials, ] ) {
@@ -464,18 +468,29 @@ def buildImages(architecture, type, boards, debosarguments = "", image = true, s
   }
 }
 
-def artifacts = [:]
+def first_pass = [:]
+def second_pass = [:]
 
 architectures.each { name, arch ->
     arch.types.each { type, params ->
         /* merge the per-arch default with per-type params */
         def merged = [:] << arch << params
-        artifacts << [("$name $type"):
-          buildImages(name, type, merged.boards, merged.args, merged.image, merged.sysroot, merged.ostree, production)
-        ]
+
+        if (!params.requires) {
+            /* first, build all jobs which don’t have any dependencies, in parallel */
+            first_pass << [("$name $type"):
+                buildImages(name, type, merged.boards, merged.args, merged.image, merged.sysroot, merged.ostree, production)
+            ]
+        } else {
+            /* second, build any jobs which depend on jobs from the first pass, also in parallel */
+            second_pass << [("$name $type"):
+                buildImages(name, type, merged.boards, merged.args, merged.image, merged.sysroot, merged.ostree, production)
+            ]
+        }
     }
 }
 
 
-parallel artifacts
+parallel first_pass
+parallel second_pass