Prepare: silence git init warning about master branch (#796)

Co-authored-by: gituser <nore@nop.np>

linux-tkg-config/prepare

@@ -429,7 +429,7 @@ _setup_kernel_work_folder() {
     msg2 "First initialization of the linux source code git folder"
     mkdir -p "$_kernel_source_folder_abs"
     cd "$_kernel_source_folder_abs"
-    git init --bare
+    git -c init.defaultBranch=master init --bare
   fi
 
   cd "$_kernel_source_folder_abs"