print usage message for build_image.sh

usr/local/bin/build_image.sh

 #!/bin/bash
 set -e
 
-img_path="$1"
-prefix="/"
-if [[ -n "$2" ]]; then
-    prefix="$2"
+if [[ $# -lt 1 ]];then
+    echo "Usage: build_image.sh <img_path> [<prefix>]"
+    exit 1
 fi
 
+img_path="$1"
+prefix=${2:-"/"}
+
 dnf -y --installroot "$img_path" --releasever=8 install system-release bash yum
 
 rsync -av "${prefix}var/local/mkosi/mkosi.extra/" "$img_path"