Add minimal STM32F103C6 support (#17853)

Unfortunately, the crippled versions of “Bluepill” boards with
STM32F103C6xx chips instead of STM32F103C8xx are now sold all over the
place, sometimes advertised in a confusing way to make the difference
not noticeable until too late.  Add minimal support for these MCUs in
the common “Bluepill with stm32duino” configuration, so that it could be
possible to make something useful from those boards (although fitting
QMK into the available 24 KiB of flash may be rather hard).

(In fact, I'm not sure whether the “STM32” part of the chip name is
actually correct for those boards of uncertain origin, so the onekey
board name is `bluepill_f103c6`; another reason for that name is to
match the existing `blackpill_f401` and `blackpill_f411`.)

The EEPROM emulation support is not included on purpose, because
enabling it without having a working firmware size check would be
irresponsible with such flash size (the chance that someone would build
a firmware where the EEPROM backing store ends up overlapping some
firmware code is really high).  Other than that, enabling the EEPROM
emulation code is mostly trivial (the `wear_leveling` driver with the
`embedded_flash` backing store even works without any custom
configuration, although its code is significantly larger than the
`vendor` driver, which may also be important for such flash size).

1 files changed, 7 insertions, 0 deletions

diff --git a/keyboards/handwired/onekey/bluepill_f103c6/readme.md b/keyboards/handwired/onekey/bluepill_f103c6/readme.md
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index 0000000000..65ba9b0d4f
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+++ b/keyboards/handwired/onekey/bluepill_f103c6/readme.md
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+# STM32F103C6xx Bluepill onekey
+
+* Supported hardware: Bluepill boards with the STM32F103C**6**xx chips
+
+Note that STM32F103C6xx chips have only 32 KiB of flash and 10 KiB of RAM, and the stm32duino bootloader occupies 8 KiB of flash, leaving only 24 KiB for the firmware, therefore the capabilities of this board are severely restricted in comparison to proper Bluepill boards with STM32F103CBxx or STM32F103C8xx chips (128 or 64 KiB of flash and 20 KiB of RAM).  You may need to disable many features in the QMK firmware to bring its compiled size below 24 KiB.  Please avoid designing new keyboards with these MCUs unless you are stuck with some hardware that you already have and don't want to just throw away.
+
+To trigger keypress, short together pins *B0* and *A7*.