Add combined DTB image tooling and DTB-agnostic GRUB configuration

[bjordiscollaku]

Summary

This PR adds end-to-end support for booting via a single combined DTB
image loaded by UEFI, and updates the Ubuntu rootfs builder to be
DTB-agnostic at the GRUB layer.

Concretely, it introduces a new build-dtb-image.sh helper for building
a FAT image containing a combined DTB, and simplifies the GRUB
configuration generated by build-ubuntu-rootfs.sh to no longer select
per-platform DTBs.

Changes

1. New DTB image tooling: build-dtb-image.sh

Adds a standalone script that generates a FAT-formatted image containing
a single combined DTB for Qualcomm-based ARM64 platforms.

Key behavior:

• Consumes a manifest listing DTB filenames (one per line), supporting:
  • Absolute paths, or
  • Paths relative to a DTB source directory (-dtb-src).
• Normalizes the manifest into a sanitized temporary list:
  • Skips blank lines and comments (#).
  • Resolves relative paths against the DTB source directory.
• Validates that:
  • The manifest file exists and yields at least one DTB entry.
  • Every referenced DTB file exists on disk.
  • The -size argument is a positive integer (MB).
• Concatenates all DTBs in manifest order into:
  • <DTB_SRC>/combined-dtb.dtb.
• Builds a FAT filesystem image:
  • Size is configurable via -size (default: 4 MB).
  • Uses dd to create the image.
  • Attaches to a loop device with losetup.
  • Formats with mkfs.vfat.
  • Mounts on a mktemp-generated directory.
  • Copies combined-dtb.dtb into the root of the FAT filesystem.

Robustness and cleanup:

• Uses set -euo pipefail.
• Installs an EXIT trap at startup.
• cleanup():
  • Preserves the original exit status.
  • Unmounts the temporary mountpoint only if it exists and is mounted.
  • Detaches the loop device only if it was created.
  • Removes the temporary manifest file and mount directory.

This provides a reproducible, automation-friendly way to package a
curated DTB set into a single combined DTB binary and corresponding FAT
image (dtb.bin) suitable for UEFI-driven boot flows and CI pipelines.

2. DTB-agnostic GRUB config in build-ubuntu-rootfs.sh

Updates the Ubuntu rootfs builder script to align with the new UEFI DTB
loading model:

• Previously:
  • GRUB generated three menu entries:
    • EVK DTB
    • CRD DTB
    • QLI/Linux-only entry
  • Each platform-specific entry referenced a DTB via a devicetree
    line, pointing into /lib/firmware/<kernel>/device-tree/....
• Now:
  • UEFI is responsible for loading dtb.bin (the combined DTB FAT
    image) before transferring control to GRUB.
  • GRUB no longer selects a DTB and does not emit any devicetree
    directives.

New behavior:

• Generates a single DTB-agnostic GRUB entry:

  set timeout=5
  
  menuentry "Ubuntu ${CODENAME}" {
      search --no-floppy --label system --set=root
      linux /boot/vmlinuz-$kernel_ver earlycon console=ttyMSM0,115200n8 \
          root=LABEL=system cma=128M rw clk_ignore_unused pd_ignore_unused \
          efi=noruntime rootwait ignore_loglevel
      initrd /boot/initrd.img-$kernel_ver
  }

[lool]

I'd suggest generating the fat as non-root; you can use mtools for that:
https://github.com/qualcomm-linux/qcom-deb-images/blob/main/debos-recipes/qualcomm-linux-debian-flash.yaml#L342

[bjordiscollaku]

I'd suggest generating the fat as non-root; you can use mtools for that: https://github.com/qualcomm-linux/qcom-deb-images/blob/main/debos-recipes/qualcomm-linux-debian-flash.yaml#L342

This commit 388f7a9 already addresses this. It was mandatory to be able to invoke the tool through a --privileged docker in workflow : )