Grub menu not found

I am working with snapshot:

Looking for recovery mode in DB820c, I found that there is neither any grub menu added on bootup nor any configurations to handle more than one recovery options.
Is there any already in-use /etc/default/grub for DB820 ??
How can we add Grub-menu to select different recovery(Firmware update,USB,SDcard,etc) purposes?
Thanks in advance…

If you want to have a rich variety of boot modes it is probably best
for you integrate u-boot since that has much richer driver support than

Thanks for the idea @danielt
Right now, I have a recovery image that follows : BL -> lk -> kernel bootup.
My approach was traversing from:
Recovery_mode -> Mounting_SDcard_&_copying_contents -> Fastboot_mode -> Flash_updated_images
To give GUI option: I was asking for a menu, so that user can choose SDboot & flash images.
If this seems achievable I would give it a shot…
Any easier/existing ways of serving this purpose are welcome…