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/test/dm/usb.c b/test/dm/usb.c
index 4300bbd..ccc07d5 100644
--- a/test/dm/usb.c
+++ b/test/dm/usb.c
@@ -5,6 +5,7 @@
  */
 
 #include <common.h>
+#include <console.h>
 #include <dm.h>
 #include <usb.h>
 #include <asm/io.h>