fix(payload): port raw external.img container

steps/improve/import_payload.sh

@@ -0,0 +1,48 @@
+#!/bin/sh
+#
+# SPDX-FileCopyrightText: 2026 live-bootstrap contributors
+# SPDX-License-Identifier: MIT
+
+set -ex
+
+if [ "${PAYLOAD_REQUIRED}" = True ]; then
+    mkdir -p /external/distfiles
+    found_payload=0
+    mkdir -p /dev
+
+    if [ ! -r /proc/partitions ]; then
+        payload-import --mount-proc >/dev/null 2>&1 || :
+    fi
+
+    if [ ! -r /proc/partitions ]; then
+        echo "payload-import failed: /proc/partitions is unavailable." >&2
+        exit 1
+    fi
+
+    while read -r major minor blocks name; do
+        case "${major}" in
+            ""|major|*[!0-9]*)
+                continue
+                ;;
+        esac
+        case "${minor}" in
+            ""|minor|*[!0-9]*)
+                continue
+                ;;
+        esac
+
+        dev_path="/dev/lbpayload-${major}-${minor}"
+        [ -b "${dev_path}" ] || mknod -m 600 "${dev_path}" b "${major}" "${minor}" >/dev/null 2>&1 || :
+
+        if payload-import --probe "${dev_path}" >/dev/null 2>&1; then
+            payload-import --device "${dev_path}" /external/distfiles
+            found_payload=1
+            break
+        fi
+    done < /proc/partitions
+
+    if [ "${found_payload}" != 1 ]; then
+        echo "payload-import failed: no payload image found in /proc/partitions devices." >&2
+        exit 1
+    fi
+fi