no_os_uart_write implementation for xilinx platform should be blocking

[buha]

no_os_uart_write should be blocking, but on xilinx platform seems it's not blocking, so some rather dirty tricks like this one were introduced to handle the inconsistency across platforms.

So:

1. make xilinx_uart_write blocking by checking the TX FIFO status, or simply using a blocking HAL function
2. remove iio_app post uart write delay, it will be useless

Probably not the highest priority change but noting it down for future reference.