Performance tuning

[laloch]

• Disable NMI watchdog - it's broken.
• Move UART ISR and FreeRTOS code to IRAM.
• Adjust UART FIFO thresholds to prevent FIFO under/overruns
• Adjust task priorities to improve throughput

This prevents data corruption caused by UART RX FIFO overruns and improves throughput in both directions.