support conditionally building artifacts in an OSBuild stage

With coreos/coreos-assembler#3930 we can
now build multiple artifacts/platforms in a single OSBuild call.
Let's add support here for:

1. Detecting if an artifact supports building via OSBuild by
   looking to see if the buildextend-{artifact} API points
   to cmd-osbuild. i.e. is OSBuild the default for this artifact?
2. Allowing the pipecfg to specify additional artifacts to build
   using OSBuild. This is useful if OSBuild is not yet the default
   way to build this particular artifact.

Author: dustymabe

config.yaml

@@ -16,6 +16,7 @@ streams:
     type: mechanical
     env:
       COSA_TESTISO_DEBUG: true
+    osbuild_artifacts: [applehv, gcp, hyperv]
   # branched:
   #  type: mechanical
   #  env:

docs/config.yaml

@@ -98,6 +98,8 @@ streams:
       check_kernel_rt_mismatch_rhcos: true
       # OPTIONAL: list of kola tags to skip for this stream
       skip_kola_tags: [openshift, needs-internet, luks]
+      # OPTIONAL: list of artifacts to additionally build using OSBuild
+      osbuild_artifacts: [applehv, gcp, hyperv]
 
 # REQUIRED: architectures to build for other than x86_64
 additional_arches: [aarch64, ppc64le, s390x]

utils.groovy

@@ -427,6 +427,26 @@ def build_artifacts(pipecfg, stream, basearch) {
     // First get the list of artifacts to build from the config
     def artifacts = get_artifacts_to_build(pipecfg, stream, basearch)
 
+    // Let's build separately the artifacts that can be built directly
+    // with OSBuild. There are two ways something can get added to the
+    // list of OSBuild built artifacts:
+    //  1. It is specified in the space separated string in env.COSA_OSBUILD_ARTIFACTS
+    //  2. If the `cmd-buildextend-{artifact}` node is a symlink to `cmd-osbuild`
+    def osbuild_artifacts = pipecfg.osbuild_artifacts ?: []
+    for (artifact in artifacts) {
+        source = "/usr/lib/coreos-assembler/cmd-buildextend-{artifact}"
+        target = shwrapCapture("cosa shell -- readlink -f ${source} | xargs basename")
+        if (target == 'cmd-osbuild') {
+            osbuild_artifacts += artifact
+        }
+    }
+    if osbuild_artifacts.length() > 0 {
+        artifacts.remove(osbuild_artifacts)
+        stage('💽:OSBuild') {
+            shwrap("cosa osbuild --platforms ${osbuild_artifacts.join(',')}")
+        }
+    }
+
     // Next let's do some massaging of the inputs based on two problems we
     // need to consider:
     //