ESP32 core v2: Two changes (1) flushes the UART TX buffer in the esp32 serial...

ESP32 core v2: Two changes (1) flushes the UART TX buffer in the esp32 serial shutdown routine. The ROM bootloader does not flush the FIFO before handing over to user code, so some of this output is not currently seen when the UART is reconfigured in early stages of startup.  And changes the openocd config file's default flash voltage from 1.8V to 3.3V. This is not necessary right now, but may save some hard-to-debug moments down the track (3.3V-only flash running at 1.8V often half-works and does weird things...)