libti*: add the ability to extract OS upgrades & boot code from TI-68k, TI-Z80, TI-eZ80 ROM dumps

[debrouxl]

Suggested by calc84maniac 2015/09/17, possibly others in the past as well.

The bulk of the code for such a feature would live in libtifiles. The implementation is quite straightforward for the TI-68k series (and probably the TI-eZ80 series, whose OS upgrades are similar), slightly less so for the TI-Z80 series.

[debrouxl]

First batch of technical notes (edited after adding the boot code extraction feature to the title):

• OS upgrade extraction:
  • there should probably be 3 functions, one for every relevant calculator series, as well as a convenience function which basically just calls the three others until one succeeds;
  • the function for the TI-Z80 series should produce at least a NULL-terminated array of FlashPage... or simply a FlashContent;
  • the functions for the TI-eZ80 series and TI-68k series could produce a { CalcModel model, uint32_t length, uint8_t * data} triplet, which should be enough... but these are part of FlashContent anyway, so let's produce a FlashContent and have a common prototype for all functions ?
  • libtifiles' intelhex code has function-static variables... probably want to fix those while at it ? intelhex.h isn't exported, so backwards compatibility won't be an impediment.
• boot code extraction:
  • likewise, a function for every calculator series, and a convenience function;
  • producing a { CalcModel model, uint32_t length, uint8_t * data } triplet should be sufficient here.