Checksum improvements

[AntiSol]

HI,

I did some work on the checksum tool, rewrote to use argparse and added a bunch of new options, e.g decimal or octal output, and the ability to append the checksum to the file (for generating USR type utilities to be run on the 1541).

I also made a tweak to the makefile to prevent creating the empty rom.bin file :)

Cheers