[coretool] add compatibility header and version info

[lydon42]

We decided to add a compatibility header to the CORE file, which will be indexed by the systems target byte to give a 0 or 1 that tells MEGAFLASH if this core is compatible with the target system, even if the core files target number is not the same as the systems. This will let us use the same core for r5 and r6 for example (currently it's two COR with the same bitstream inside).

We will also add a CORE Version info to the header for further expansions and changes.

Currently:

• V1: old format, has no length nor CRC32 (all zero or wrong)
• V2: current format, has length and CRC32, but a zero version.
• V3: new format, header has the core version of 3 or higher.

[lydon42]

Probably we don't need to add the compatibility header, as the minor version will solve that problem. And only for R5 it does not make sense to add this.

Adding a core version is still a good idea.