Pass disk context to GRUB

Hello,

I'm trying to enable UEFI dual-booting on the BeagleBone black. U-Boot knows to boot from SD if present, and eMMC if not. However, U-Boot doesn't pass the disk "context" to GRUB; GRUB always thinks it's booting from `hd0`, and doesn't know what to use for the kernel's `root` parameter.

I checked the UEFI API, and it seems U-Boot passes a handle to the disk that the GRUB EFI loader was loaded from. Is it possible to also pass a value that can be used to determine the kernel's `root` based on if we're booting from SD or eMMC (and in the future, USB)?

For more context, see https://github.com/mendersoftware/meta-mender/pull/1010#issuecomment-6471724... .

Thank you, Garrett