Synchronize all invokes of uart_puts()

[kbabioch]

Commit 93a6024 has introduced the problem that most of the logging output does not longer work as expected due to the fact that the output buffer is not big enough to hold all of the data being output.

Although the output buffer could be increased, it is still necessary to synchronize the output by invoking uart_flush() from time to time.

[kbabioch]

Making uart_putc() block in case the appropriate FIFO is full might be a better idea, but might potentially lead to deadlock situation.