Fix the issue "fatal: Not a git repository" for reclass-system

 git submodule add requires relative path to the end dir.

Change-Id: I6f5fe246013bddce20cfe44600ce148cca1a53b1
diff --git a/generate-cookiecutter-products.groovy b/generate-cookiecutter-products.groovy
index 02e68d0..39590e5 100644
--- a/generate-cookiecutter-products.groovy
+++ b/generate-cookiecutter-products.groovy
@@ -96,7 +96,7 @@
                 dir(path: modelEnv) {
                     sh "rm -rfv .git"
                     sh "git init"
-                    ssh.agentSh("git submodule add ${sharedReclassUrl} ${systemEnv}")
+                    ssh.agentSh("git submodule add ${sharedReclassUrl} 'classes/system'")
                 }
                 def sharedReclassBranch = templateContext.default_context.shared_reclass_branch
                 // Use refspec if exists first of all