image-info: move image opening to own function

Move the code of the current main into is own method and call that
from main. This prepares support for analyzing other types than
images. Additionally, add argument parsing via argparse to get a
help texts.

tools/image-info

@@ -1,5 +1,6 @@
 #!/usr/bin/python3
 
+import argparse
 import contextlib
 import glob
 import json
@@ -255,7 +256,9 @@ def find_esp(partitions):
     return None, 0
 
 
-def main(image):
+def analyse_image(image):
+    subprocess.run(["modprobe", "nbd"], check=True)
+
     report = {}
     with nbd_connect(image) as device:
         report["image-format"] = read_image_format(image)
@@ -279,8 +282,17 @@ def main(image):
 
     return report
 
-if __name__ == "__main__":
-    subprocess.run(["modprobe", "nbd"], check=True)
 
-    report = main(sys.argv[1])
+def main():
+    parser = argparse.ArgumentParser(description="Inspect an image")
+    parser.add_argument("target",  help="The file or directory to analyse")
+
+    args = parser.parse_args()
+    report = analyse_image(args.target)
+
     json.dump(report, sys.stdout, sort_keys=True, indent=2)
+
+
+if __name__ == "__main__":
+    main()
+