serial: ns16550: fix debug uart putc called before init

serial: ns16550: fix debug uart putc called before init

If _debug_uart_putc() is called before _debug_uart_init(), the
ns16550 debug uart driver hangs in a tight loop waiting for the
tx FIFO to get empty.

As this can happen via a printf sneaking in before the port calls
debug_uart_init(), introduce a config option to ignore characters
before the debug uart is initialized.

This is done by reading the baudrate divisor and aborting if is zero.

The Kconfig option is required as reading the baudrate divisor does
not seem to work for all ns16500 compatibles (which is why the last
attempt on this has been reverted in 1a67969a99).

Tested on socfpga_cyclone5_socrates.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>