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/cmd_mem.c b/common/cmd_mem.c
index 43c3fb6..9fb2584 100644
--- a/common/cmd_mem.c
+++ b/common/cmd_mem.c
@@ -12,9 +12,11 @@
  */
 
 #include <common.h>
+#include <console.h>
 #include <bootretry.h>
 #include <cli.h>
 #include <command.h>
+#include <console.h>
 #ifdef CONFIG_HAS_DATAFLASH
 #include <dataflash.h>
 #endif