kernel-dtb: surface skipped preprocessed-DTS step in output (fixes #8083)

When BOOT_FDT_FILE is unset, kernel_dtb_only_build() still produces the
linux-dtb .deb (from `make dtbs` + dtbs_install) but the convenience
preprocessed-DTS copy into output/ is skipped. The previous warn-level
message was easy to miss, which is what led to the original report of
"no output of the preprocessed dts files".

Change the skip message to err-level and spell out that the package
itself is fine and point at both likely config sources.

Author: iav

lib/functions/compilation/kernel.sh

@@ -183,9 +183,8 @@ 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 '${BRANCH}'" "No BOOT_FDT_FILE set for board, skipping further processing" "warn"
+		display_alert "Board '${BOARD}' branch '${BRANCH}' has no BOOT_FDT_FILE" "linux-dtb package produced OK; preprocessed DTS copy skipped. Check board config and any post_family_config_* hook that may unset it." "err"
 		return 0
 	fi