Add grubcustom manually to prevent stuck on GRUB menu when using BOSS-S1 RAID card.

[masteryyh]

Problem:
We might stuck in Welcome to GRUB! message for ~30min when installing Harvester on baremetal machines with Dell's BOSS-S1 RAID card.

Solution:

Add /oem/grubcustom manually to prevent long time search for this file when booting up.

Related Issue:
harvester/harvester#4334

Test plan:

1. Find a server with BOSS-S1 card, and some M.2 disks installed on the card;
2. Build an ISO using this branch;
3. Use the ISO just built to install Harvester on this server, must be installed on the disk installed on BOSS-S1 card;
4. Boot up the machine, and it should not stuck on Welcome to GRUB! for too long.

Additional context:
If you want to test this on Equinix Metal machines, here is the step:

1. Find an environment with public IP address and build/put image from this branch, and serve the image using nginx;
2. Use terraform scripts here, edit variables.tf, fill in the project name and version (should be master), set node number to 1, choose server location that close to you (e.g HK or SG), set plan to m3.large.x86;
3. Create ipxe script, you can copy one from existing scripts, change image url to your image http server, and put this script to your http server too;
4. Change ipxe_script url to your http server;
5. Apply this terraform configuration according to README.md, and you should see a new server created in Equinix Metal dashboard;
6. Use Out-of-Band SSH console to see serial output. It should works just like KVM remote console.

[masteryyh]

I tested on Equinix Metal machines and this fix should works ;)

[bk201]

LGTM!

[Vicente-Cheng]

LGTM, thanks for fixing that!

[guangbochen]

LGTM, thanks.