Switch the plugin to use the `F303CC_GENERIC` variant rather than `F303ZE_EVAL`,
to match the hardware more closely. Also drop the keyscanner, we do not need
that for most testing, and it was just complicating things.
On top of those simplifications, add a `serialPort()` method to make
`FocusSerial` work, and `rebootBootloader()` to make rebooting function as one
would expect.
Signed-off-by: Gergely Nagy <algernon@keyboard.io>