CARTRIDGE: define a MEGA65 style cartridge

[lydon42]

The MEGA65 has special code for handling a C64 style cartridge and ensuring that it is started when detected. This was recently expanded by letting the slot 0 cores MEGAFLASH decide which core slot to start, depending on the type of cartridge detected.

This brings up the need to define a pattern by which a MEGA65 style cartridge can be detected, so that a MEGA65 core is started, as all the default /EXROM and /GAME settings should bring up a C64 core (if the user chooses this to be their default).

There are two possible patterns that come to mind naturally:

• C128 style: EXROM/GAME are not pulled down, the ROM needs to check memory for a special sequence (it's CBM for the C128, we can use M65 for the MEGA65), and then map or DMA copy this memory and call special setup and start vectors
• C64 style: use EXROM/GAME in Ultimax mode (that replaces the ROM, so it will start automatically) and just add a marker (let's do it C128 style again) so the core can detect that this is a non C64 cartridge, even if it seems to be Ultimax

As the Ultimax approach is kind of tedious (we stay in C64 personality, only RAM below $1000 is accessible), we implement both approaches for now, so that the slot 0 core will detect both types and start a MEGA65 core if it does.

[lydon42]

The definition how the cartridge is detected is done, and basically also the startup implementation in ROM.

The important part for Batch 3 is that MEGAFLASH can detect and choose the right core, everything that happens afterwards does not need to go into the Factory Core, so we are done with that part for now.