Restore some default behaviors

- If `init` is not specified, wwinit looks for /sbin/init, /etc/init,
  and /bin/init, matching dracut behavior.
- assume ipxe template=default if not set
- Display a warning during overlay build if an overlay list is empty

Issues:

- Closes: #1808
- Closes: #1813

Signed-off-by: Jonathon Anderson <janderson@ciq.com>

internal/pkg/warewulfd/provision.go

@@ -97,7 +97,11 @@ func ProvisionSend(w http.ResponseWriter, req *http.Request) {
 		}
 
 	} else if rinfo.stage == "ipxe" {
-		stage_file = path.Join(conf.Paths.Sysconfdir, "warewulf/ipxe/"+remoteNode.Ipxe+".ipxe")
+		template := remoteNode.Ipxe
+		if template == "" {
+			template = "default"
+		}
+		stage_file = path.Join(conf.Paths.Sysconfdir, "warewulf/ipxe", template+".ipxe")
 		kernelArgs := ""
 		kernelVersion := ""
 		if remoteNode.Kernel != nil {