IOMUX: Add console multiplexing support.

Modifications to support console multiplexing.  This is controlled using
CONFIG_SYS_CONSOLE_MUX in the board configuration file.

This allows a user to specify multiple console devices in the environment
with a command like this: setenv stdin serial,nc.  As a result, the user can
enter text on both the serial and netconsole interfaces.

All devices - stdin, stdout and stderr - can be set in this manner.

1) common/iomux.c and include/iomux.h contain the environment setting
implementation.
2) doc/README.iomux contains a somewhat more detailed description.
3) The implementation in (1) is called from common/cmd_nvedit.c to
handle setenv and from common/console.c to handle initialization of
input/output devices at boot time.
4) common/console.c also contains the code needed to poll multiple console
devices for input and send output to all devices registered for output.
5) include/common.h includes iomux.h and common/Makefile generates iomux.o
when CONFIG_SYS_CONSOLE_MUX is set.

Signed-off-by: Gary Jennejohn <garyj@denx.de>

include/common.h

@@ -678,6 +678,13 @@ void	fputc(int file, const char c);
 int	ftstc(int file);
 int	fgetc(int file);
 
+/*
+ * CONSOLE multiplexing.
+ */
+#ifdef CONFIG_CONSOLE_MUX
+#include <iomux.h>
+#endif
+
 int	pcmcia_init (void);
 
 #ifdef CONFIG_STATUS_LED