postinst: skip updating bootloader configuration on new GRUB systems

Here is the diff ignoring the indentation change:

```patch
diff --git a/coreos-postinst b/coreos-postinst
index 2161aad..33ab39b 100644
--- a/coreos-postinst
+++ b/coreos-postinst
@@ -23,6 +23,8 @@ case "${INSTALL_LABEL}" in
         exit 1
 esac
 
+# Update bootloaders from CoreOS <= 522.x.x
+if grep -q cros_legacy /proc/cmdline; then
     # Find the ESP partition and mount it if needed
     ESP_PARTTYPE="c12a7328-f81f-11d2-ba4b-00a0c93ec93b"
     ESP_MNT=
@@ -74,6 +76,7 @@ if ! grep -q boot_kernel "${ESP_MNT}/syslinux/default.cfg"; then
         cp -v "${INSTALL_MNT}/boot/syslinux/default.cfg.${SLOT}" \
             "${ESP_MNT}/syslinux/default.cfg"
     fi
+fi
 
 # If the OEM provides a hook, call it
 if [[ -x "${OEM_MNT}/bin/oem-postinst" ]]; then
```