kernel-install: Use "Default" as fallback instead of "Linux"

"Linux" conflicts /efi/Linux when /efi is the install location.
/efi/Linux is already reserved for unified kernel images so we can't use
it for type #1 images. Instead, we use "Default".

src/kernel-install/kernel-install

@@ -88,7 +88,7 @@ KERNEL_IMAGE="$2"
 if [[ -f /etc/machine-id ]]; then
     read MACHINE_ID < /etc/machine-id
 else
-    MACHINE_ID="Linux"
+    MACHINE_ID="Default"
 fi
 
 if [[ ! $COMMAND ]] || [[ ! $KERNEL_VERSION ]]; then