serial/ns16550: add an option to avoid hanging on broken platforms

Some platforms (e.g. IGEPv2 board) has a broken ns16550 UART that
does not set the TEMT bit when the transmitter is empty in SPL.
This makes U-Boot to hang while waiting for TEMT to be set.

Add a new option to avoid this:

CONFIG_SYS_NS16550_BROKEN_TEMT

16550 UART set the Transmitter Empty (TEMT) Bit when all output
has finished and the transmitter is totally empty. U-Boot waits
for this bit to be set to initialize the serial console. On some
broken platforms this bit is not set in SPL making U-Boot to
hang while waiting for TEMT. Define this option to avoid it.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>

README

@@ -616,6 +616,14 @@ The following options need to be configured:
 		boot loader that has already initialized the UART.  Define this
 		variable to flush the UART at init time.
 
+		CONFIG_SYS_NS16550_BROKEN_TEMT
+
+		16550 UART set the Transmitter Empty (TEMT) Bit when all output
+		has finished and the transmitter is totally empty. U-Boot waits
+		for this bit to be set to initialize the serial console. On some
+		broken platforms this bit is not set in SPL making U-Boot to
+		hang while waiting for TEMT. Define this option to avoid it.
+
 
 - Console Interface:
 		Depending on board, define exactly one serial port

drivers/serial/ns16550.c

@@ -36,8 +36,10 @@
 
 void NS16550_init(NS16550_t com_port, int baud_divisor)
 {
+#if (!defined(CONFIG_SYS_NS16550_BROKEN_TEMT))
 	while (!(serial_in(&com_port->lsr) & UART_LSR_TEMT))
 		;
+#endif
 
 	serial_out(CONFIG_SYS_NS16550_IER, &com_port->ier);
 #if (defined(CONFIG_OMAP) && !defined(CONFIG_OMAP3_ZOOM2)) || \