i2c: revise timing calculation

[flannelhead]

Look-up tables were used for determining the delay loop counts before. Based on these hand-tuned values, the loop overhead was estimated for each option -- 80 and 160 MHz, fast and slow GPIO access. Instead of the great number of tunable parameters one now only has to tune the overhead values if the code is changed.

Functions were added to the API which allow setting an arbitrary frequency. API backward compatibility is retained.

[Zaltora]

You tested if frequency signals are good for each option and frequency ? Nice update. I guess, this soft i2c library will be the best of the esp8266 ^^

[flannelhead]

@Zaltora: I tested with some configurations. Probably I should still check all the combinations (80MHz/register access, 80MHz/GPIO API, 160MHz/register access and 160MHz/GPIO API).

However, the formula that was introduced here produces the very same delay values that were hand-tuned and hardcoded before (if not even more precise!) The linear delay formula and coefficients are based on those hand-tuned values but the number of parameters is reduced.

[UncleRus]

Thank you!