Move console definitions into a new console.h file

The console includes a global variable and several functions that are only
used by a small subset of U-Boot files. Before adding more functions, move
the definitions into their own header file.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/console.c b/common/console.c
index ace206c..10972b0 100644
--- a/common/console.c
+++ b/common/console.c
@@ -6,6 +6,7 @@
  */
 
 #include <common.h>
+#include <console.h>
 #include <debug_uart.h>
 #include <stdarg.h>
 #include <iomux.h>