README: Remove references to "make axtls", it's no longer needed.
README: Remove text about selecting different ports in the docs.
py/unicode: Fix check for valid utf8 being stricter about contn chars.
stm32/adc: Add ADC auto-calibration for L4 MCUs.
stm32/flashbdev: Add missing include for irq.h.
tests/io: Update tests to use uos.remove() instead of uos.unlink().
tools/pydfu.py: Improve DFU reset, and auto-detect USB transfer size.
stm32/servo: Only initialise TIM5 if it is needed, to save power.