Beautiful boot screen

Boot program has been changed to show less garbage on screen while switching to c64 mode, making booting look a bit neater. The second bootblock now loads at $0C00 and therefore tends to stick around a bit longer than if loaded directly into screen memory at $0400. Now you can SYS3072 to switch to c64 mode and run the program.

Also, code has been cleaned up and now uses more common labels for well known addresses.