Too small EEPROM size partitioning on K20x boards

[firetech]

(Bringing findings from discussions in #12947 and #9657 to one place.)

Boards based on a K20x MCU currently get their "FlexMemory" partitioned as a 32 byte EEPROM by default, unless partitioned by something else than QMK before. This is becoming an issue since 32 bytes is less than the current EECONFIG_SIZE (35), and out-of-bounds read/write attemps are just ignored in that EEPROM driver. Case in point.

This partitioning can only be changed after a "clear all blocks" command, which will wipe the entire MCU flash. For Teensy boards, this isn't a problem since their bootloader resides in an external chip, but for (at least) Input:Club keyboards, which are using K20x MCUs with the bootloader residing in the MCU's program flash, this would require reflashing the bootloader using e.g. a J-link or Bus Pirate. One thing to note is that the Kiibohd firmware for these boards doesn't seem to use the EEPROM at all, so it's most probably QMK that has partitioned these boards to the 32 byte size.

Just increasing the default EEPROM_SIZE will cause issues for already partitioned boards, since out-of-bounds (read?/)write attempts will cause the EEERDY flag to stay low and therefore causes an infinite loop in flexram_wait().

One way to solve this is to check what size EEPROM the MCU has been set to (in the SIM->FCFG1 register) before the first EEPROM access, and adapt from that size so out-of-bounds accesses are avoided. One can also go further and switch the FlexRAM over to act as traditional RAM (making the same EEPROM driver act as 2048 bytes of transient storage instead) if the partitioned EEPROM size is deemed too small, but that is optional. I have made a working prototype of this behaviour (this block is the optional extra step of going transient), and if that is deemed an adequate solution to the issue, I'll make a PR of this after #12947 has been merged (would cause lots of merge conflicts otherwise).

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