40ignition-conf: add new base config

Ignition now supports a `base.d` directory for base config fragments that allows FCOS and
RHCOS to maintain the same `00-core.ign`, and facilitate the distro-specific changes by placing
the other base config fragments in a separate `overlay` directory(coreos/ignition#1108).
This commit places 00-core.ign in a `base.d` directory.

Also, keeping the legacy base config and the Afterburn bit as-is, otherwise, it will break CI.
Once we have the new Ignition release, this PR (#717) will remove the legacy base config and revert #626

overlay.d/05core/usr/lib/dracut/modules.d/40ignition-conf/00-core.ign

@@ -0,0 +1,19 @@
+{
+  "ignition": {
+    "version": "3.0.0"
+  },
+  "passwd": {
+    "users": [
+      {
+        "name": "core",
+        "gecos": "CoreOS Admin",
+        "groups": [
+          "adm",
+          "sudo",
+          "systemd-journal",
+          "wheel"
+        ]
+      }
+    ]
+  }
+}

overlay.d/05core/usr/lib/dracut/modules.d/40ignition-conf/module-setup.sh

@@ -7,6 +7,9 @@ depends() {
 }
 
 install() {
+    mkdir -p "$initdir/usr/lib/ignition/base.d"
+    inst "$moddir/00-core.ign" \
+        "/usr/lib/ignition/base.d/00-core.ign"
     inst "$moddir/base.ign" \
         "/usr/lib/ignition/base.ign"
 }