rancher-turtles: Fix IMG_PREFIX macros

Currently the substitution is not working correctly

metallb-chart/values.yaml

@@ -59,7 +59,7 @@ prometheus:
   # the image to be used for the kuberbacproxy container
   rbacProxy:
     repository: "%%IMG_REPO%%/%%IMG_PREFIX%%kube-rbac-proxy"
-    tag: "0.18.1"
+    tag: "v0.18.0"
     pullPolicy: IfNotPresent
 
   # Prometheus Operator PodMonitors

rancher-turtles-airgap-resources-chart/_service

@@ -2,7 +2,7 @@
   <service mode="buildtime" name="kiwi_metainfo_helper"/>
   <service name="replace_using_env" mode="buildtime">
     <param name="file">Chart.yaml</param>
-    <param name="eval">IMG_PREFIX=$(rpm --macros=/root/.rpmmacros -E %img_prefix)</param>
+    <param name="eval">IMG_PREFIX=$(rpm --macros=/root/.rpmmacros -E %{?img_prefix})</param>
     <param name="var">IMG_PREFIX</param>
   </service>
 </services>

rancher-turtles-chart/_service

@@ -2,14 +2,14 @@
   <service mode="buildtime" name="kiwi_metainfo_helper"/>
   <service name="replace_using_env" mode="buildtime">
     <param name="file">values.yaml</param>
-    <param name="eval">IMG_PREFIX=$(rpm --macros=/root/.rpmmacros -E %img_prefix)</param>
+    <param name="eval">IMG_PREFIX=$(rpm --macros=/root/.rpmmacros -E %{?img_prefix})</param>
     <param name="var">IMG_PREFIX</param>
     <param name="eval">IMG_REPO=$(rpm --macros=/root/.rpmmacros -E %img_repo)</param>
     <param name="var">IMG_REPO</param>
   </service>
   <service name="replace_using_env" mode="buildtime">
     <param name="file">Chart.yaml</param>
-    <param name="eval">IMG_PREFIX=$(rpm --macros=/root/.rpmmacros -E %img_prefix)</param>
+    <param name="eval">IMG_PREFIX=$(rpm --macros=/root/.rpmmacros -E %{?img_prefix})</param>
     <param name="var">IMG_PREFIX</param>
   </service>
 </services>