kernel-dtb: fatal if BOOT_FDT_FILE is unset (fixes #8083)

kernel-dtb ships a preprocessed .dts for the board's active device
tree, and it needs BOOT_FDT_FILE to know which .dtb to process. When
the board config (or a post_family_config_* hook) leaves BOOT_FDT_FILE
unset, the old code emitted a warn alert and returned, silently
producing an empty package — easy to miss in a long build log.

Turn this into a fatal error inside kernel_dtb_only_build(). The
function is only reached from kernel_prepare_build_and_package() when
KERNEL_DTB_ONLY=yes, so regular 'kernel' builds (which do ship DTBs,
just without the preprocessed .dts convenience) are unaffected.

Assisted-by: Claude:claude-opus-4-7

Author: iav

lib/functions/compilation/kernel.sh

@@ -174,6 +174,11 @@ function kernel_prepare_build_and_package() {
 
 function kernel_dtb_only_build() {
 	display_alert "Kernel DTB-only for development" "KERNEL_DTB_ONLY: ${KERNEL_DTB_ONLY}" "info"
+
+	if [[ -z "${BOOT_FDT_FILE}" ]]; then
+		exit_with_error "Board '${BOARD}' branch '${BRANCH}' has no BOOT_FDT_FILE set; kernel-dtb cannot produce a preprocessed DTS. Check the board config and any post_family_config_* hook that may unset it."
+	fi
+
 	# Do it in two separate steps, first build the dtbs then install them.
 	build_targets_build=("dtbs")
 	build_targets_install=("dtbs_install")
@@ -183,12 +188,6 @@ function kernel_dtb_only_build() {
 
 	display_alert "Considering further .dts convenience processing" "for board '${BOARD}' branch '${BRANCH}'" "info"
 
-	# If BOOT_FDT_FILE is not set, bail.
-	if [[ -z "${BOOT_FDT_FILE}" ]]; then
-		display_alert "Board '${BOARD}' branch '${BRANCH}'" "No BOOT_FDT_FILE set for board, skipping further processing" "warn"
-		return 0
-	fi
-
 	display_alert "Kernel DTB-only for development" "Copying preprocessed versions of ${BOOT_FDT_FILE}" "info"
 
 	declare fdt_dir fdt_file                 # we need to parse these out of BOOT_FDT_FILE