fix: use kernel-built DTBs for RPi SD card driver probe

The sdhci-iproc driver (RPi 4 SD card controller) probes via Device
Tree matching. Using DTBs from the firmware repo instead of the
kernel build caused a mismatch — the driver silently failed to probe,
resulting in zero block devices after boot.

Changes:
- Use DTBs from custom-kernel-arm64/dtbs/ (matches the kernel)
- Firmware blobs (start4.elf, fixup4.dat) still from firmware repo
- Also includes prior fix for LABEL= resolution in persistent mount

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

build/scripts/create-rpi-image.sh

@@ -34,6 +34,9 @@ IMG_SIZE_MB="${IMG_SIZE_MB:-2048}"  # 2 GB default
 KERNEL="${CACHE_DIR}/custom-kernel-arm64/Image"
 INITRAMFS="${ROOTFS_DIR}/kubesolo-os.gz"
 RPI_FIRMWARE_DIR="${CACHE_DIR}/rpi-firmware"
+# DTBs MUST come from the kernel build (not firmware repo) to match the kernel.
+# A DTB mismatch causes sdhci-iproc to silently fail — zero block devices.
+KERNEL_DTBS_DIR="${CACHE_DIR}/custom-kernel-arm64/dtbs"
 
 echo "==> Creating ${IMG_SIZE_MB}MB Raspberry Pi disk image..."
 
@@ -178,14 +181,15 @@ CFGTXT
     # Copy initramfs
     cp "$INITRAMFS" "$MNT/kubesolo-os.gz"
 
-    # Copy DTB overlays
-    if [ -d "$RPI_FIRMWARE_DIR/overlays" ]; then
-        cp -r "$RPI_FIRMWARE_DIR/overlays" "$MNT/"
+    # Copy DTBs from kernel build (MUST match kernel to avoid driver probe failures)
+    if ls "$KERNEL_DTBS_DIR"/bcm27*.dtb 1>/dev/null 2>&1; then
+        cp "$KERNEL_DTBS_DIR"/bcm27*.dtb "$MNT/"
     fi
-
-    # Copy base DTBs (bcm2710-*, bcm2711-*, bcm2712-*)
-    if ls "$RPI_FIRMWARE_DIR"/bcm27*.dtb 1>/dev/null 2>&1; then
-        cp "$RPI_FIRMWARE_DIR"/bcm27*.dtb "$MNT/"
+    # Copy overlays — prefer kernel-built, fall back to firmware repo
+    if [ -d "$KERNEL_DTBS_DIR/overlays" ]; then
+        cp -r "$KERNEL_DTBS_DIR/overlays" "$MNT/"
+    elif [ -d "$RPI_FIRMWARE_DIR/overlays" ]; then
+        cp -r "$RPI_FIRMWARE_DIR/overlays" "$MNT/"
     fi
 
     # Write version marker