common: move extern char console_buffer[] to common.h

Extract all extern declarations for console_buffer[] out of c files into
the common.h header.

Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
Cc: Frank Gottschling <fgottschling@eltec.de>
Cc: Murray Jensen <Murray.Jensen@csiro.au>

include/common.h

@@ -272,6 +272,7 @@ void	reset_cmd_timeout(void);
 #ifdef CONFIG_MENU
 int	abortboot(int bootdelay);
 #endif
+extern char console_buffer[];
 
 /* arch/$(ARCH)/lib/board.c */
 void	board_init_f  (ulong) __attribute__ ((noreturn));