scripts: add build-dtb-image.sh — self-contained FIT DTB image builder

Introduce build-dtb-image.sh, a production-ready tool for generating
FAT-formatted FIT DTB images for Qualcomm ARM64 platforms.

The script is co-located with the metadata it consumes (qcom-metadata.dts,
qcom-next-fitimage.its), eliminating the need to clone this repository
at build time.  The metadata directory is resolved at runtime via
BASH_SOURCE[0], so the script works correctly regardless of the caller's
working directory or CI environment.

FIT image mode is the default and only operating mode.  The --fit-image
flag is accepted for backward compatibility with existing callers but is
a no-op.

Two DTB source sub-modes are supported:

  --kernel-deb  Extract DTBs from a Debian kernel package (.deb).
                Probes paths in order:
                  1. usr/lib/linux-image-*/        (Debian standard)
                  2. usr/lib/firmware/*/device-tree (Ubuntu, usrmerge)
                  3. lib/firmware/*/device-tree     (Ubuntu, legacy)
                This probe order ensures correct operation on both Debian
                and Ubuntu regardless of usrmerge layout, and is fully
                backward compatible with legacy build-kernel-deb.sh packages.

  --dtb-src     Read DTBs directly from a kernel build tree directory.

Build pipeline (5 steps):
  1. Assemble a temporary staging tree with ITS, compiled metadata DTB,
     and all per-platform DTBs laid out as the ITS /incbin/ paths expect.
  2. Compile qcom-metadata.dts → qcom-metadata.dtb via dtc.
  3. Copy qcom-next-fitimage.its into the staging directory.
  4. Invoke mkimage from the staging directory to produce qclinux_fit.img
     (-E -B 8).  The output filename is hardcoded in UEFI firmware and
     must not be changed.
  5. Pack qclinux_fit.img into a FAT image (default: dtb.bin, 4 MB).

Hardening:
  - Upfront validation that qcom-metadata.dts and the ITS file exist
    before any build work begins.
  - find -L replaces flat *.dtb* glob, correctly collecting DTBs nested
    under vendor subdirectories (e.g. qcom/).
  - Staged DTB count check with sorted listing for early diagnosis of
    empty or misconfigured DTB sources.
  - Cleanup trap covers all temporary resources (loop device, mount
    point, .deb extraction dir, staging dir) on any exit path.
Signed-off-by: Bjordis Collaku <bcollaku@qti.qualcomm.com>

Author: bjordiscollaku