refactor+docs(payload.img, payload.img discovery): split offline distfiles at improve: import_payload so main image is minimal and payload.img is primary carrier, detect payload.img automaticly using magic number

steps/improve/import_payload.sh

@@ -7,5 +7,31 @@ set -ex
 
 if [ "${PAYLOAD_REQUIRED}" = True ]; then
     mkdir -p /external/distfiles
-    payload-import /external/distfiles
+    found_payload=0
+
+    # Probe all block devices reported by the running kernel instead of
+    # assuming fixed names like /dev/sdb or /dev/hdb.
+    while read -r major minor blocks name; do
+        case "${name}" in
+            ""|name|ram*|loop*|fd*|sr*|md*)
+                continue
+                ;;
+        esac
+
+        dev_path="/dev/${name}"
+        if [ ! -b "${dev_path}" ]; then
+            mknod -m 600 "${dev_path}" b "${major}" "${minor}" || :
+        fi
+
+        if payload-import --probe "${dev_path}"; 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 on detected block devices." >&2
+        exit 1
+    fi
 fi