In dbgblock.py, renamed -x/--cksum -> -c/--cksum

I think this makes a bit more sense.

I think the original reasoning for -x/--cksum was to match -x/--device
in dbgrbyd.py, but that flag no longer exists. This could go all the way
back to matching --xsum at some point, but I'm not sure.

Common hash related utils, sha256sum, md5sum, etc, use -c/--check to
validate their hash, so that's sort of prior art?

scripts/dbgblock.py

@@ -179,7 +179,7 @@ if __name__ == "__main__":
             for x in x.split(',')),
         help="Show this many bytes, may be a range.")
     parser.add_argument(
-        '-x', '--cksum', '--crc32c',
+        '-c', '--cksum', '--crc32c',
         action='store_true',
         help="Calculate and show the crc32c of the data.")
     sys.exit(main(**{k: v