Add STM32F411 configuration support - #1

[YanceyA]

Summary

• add a STM32F411 selection with tailored clock, memory, and MCU defaults so menuconfig emits stm32f411xe
• update the STM32F4 clock, ADC, and PWM logic for the STM32F411 while guarding pins absent from the Blackpill package
• allow flash_usb.py to recognize STM32F411 builds and document the usual DFU/serial USB identifiers
• default the STM32F411 clock reference to 25 MHz and hide GPIOE pin entries so menuconfig reflects the Blackpill package pinout

Testing

• python -m compileall scripts/flash_usb.py

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https://chatgpt.com/codex/tasks/task_e_6900223348c0833384271c49ed8e6112