Add build-dtb-image.sh for combined DTB FAT image generation #234

Job	Run time
qcom-preflight-checks / semgrep/ci	1m 37s
qcom-preflight-checks / repolinter	33s
qcom-preflight-checks / dependency-review	6s
qcom-preflight-checks / copyright-license-check	52s
qcom-preflight-checks / commit-emails-check	7s
	3m 15s

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