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/command.c b/common/command.c
index 972ae28..858e288 100644
--- a/common/command.c
+++ b/common/command.c
@@ -11,6 +11,7 @@
 
 #include <common.h>
 #include <command.h>
+#include <console.h>
 #include <linux/ctype.h>
 
 /*