Switch base_dir as null on centos, keep var in place

Closes https://github.com/netbootxyz/netboot.xyz/pull/1565

roles/netbootxyz/defaults/main.yml

@@ -176,7 +176,7 @@ releases:
     - code_name: current
       name: current
   centos:
-    base_dir: centos-stream
+    base_dir: null
     enabled: true
     menu: linux
     mirror: https://mirror.stream.centos.org

roles/netbootxyz/templates/menu/centos.ipxe.j2

@@ -20,7 +20,7 @@ item {{ item.code_name }} ${space} ${os} {{ item.name }}
 {% endfor %}
 isset ${osversion} || choose osversion || goto linux_menu
 echo ${cls}
-set dir ${osversion}/BaseOS/${os_arch}/os ||
+set dir ${centos_base_dir}/${osversion}/BaseOS/${os_arch}/os ||
 set repo ${centos_mirror}/${dir}
 goto boottype