uart_send_char example not working

This example has been reported broken. The issue is probably the initialization of the pads in the PMSIS library included in the docker image used for building. In this the pad configuration for the UART has been switched off to free up the UART for use on other decks. The preferred way to communicate with the Crazyflie STM32 is via CPX though the ESP32 (examples/other/stm_gap8_cpx).