Fix UART error IRQs not being cleared
Detail: s/UART - Update HAL_UARTReceiveByte to clear error IRQs when the FIFO is believed to be empty. Disable IRQs in some complex routines to prevent any re-entrancy issues, and also clear the RX threshold IRQ when flushing the FIFOs or adjusting the threshold level. Admin: Tested on Raspberry Pi 1 Resolves ticket #429 Version 0.63. Tagged as 'BCM2835-0_63'
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