Recovery Ubuntu boot entry in UEFI after upgraded motherboard firmware

Nowadays, UEFI is commonly used to replace the BIOS. UEFI doesn't use boot sector in disks. Instead, UEFI comes with a boot manager. During installation of an OS, e.g. Ubuntu Linux, the OS installer usually setup the boot loader (e.g. GRUB) in EFI partition and update the boot config. The boot config provides information for boot manager to locate the boot loader. The tricky part is the boot config is stored in NVRAM on the motherboard. During firmware upgrade, NVRAM can be reset. This means the boot config will also be reset and lost the boot loader information.
I encountered this issue when I upgrade the firmware of my Xubuntu box. Now, I would like to show you how to recovery the boot config. This demo is using Ubuntu 16.04 on Virtual Box.

1.  Get an installation CD or a flash drive with installer of Ubuntu 16.04 (You should have it when you install the Ubuntu first time). If you don't have it, get it from here.
2.  Boot up the system using the CD or flash drive. In GRUB, select "Try Ubuntu without installation"
3. Then, you should be able to boot into a fresh Ubuntu, like
   
4. Open a terminal and run "sudo su" to switch to root user.
5. Now, we need to know where the EFI partition located. We can use "fdisk -l /dev/xxx" command to list out the paration, like:
   Refer to the above screen, we know that the EFI partition is on the first partition of device "sda" (i.e. /dev/sda1). (Note: the actual location of the EFI partition on your box can be different from this demo.)
6. Then, we mount the EFI partition to verify if the Ubuntu GRUB loader is installed. To do this, mount the drive using command "udiskctrl mount -b /dev/xxx" and check if the EFI partition contains "EFI/ubuntu/shimx64.efi" file. E.g.:
   
7. After the above verification, we have enough information to create the add the boot entry using the command:
   efibootmgr -c -d /dev/xxx -p n -l '\EFI\ubuntu\shimx64.efi'
   where "xxx" is the disk device contains the EFI partition, and "n" is the partition number. E.g.:
   

shutdown the box, remove the CD or flash drive and reboot. Then, we should be able to see the GRUB again.