[installer] Use the Grub version comes from ONL OS

[david56]

The Grub in ONiE OS is always out of date. Reinstalling ONiE and ONL for upgrading Grub is quite complex and takes a lot of time.

In ONL runtime, it can upgrade the Grub directly by performing grub-install command with appropriate arguments.

This patch changes the ONL installer to include libraries come from ONL root filesystem for grub-install. A fresh installed ONL will have the Grub version from ONL OS immediately.