common: Drop bootstage.h from common header

Move this fairly uncommon header out of the common header.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/disk.c b/cmd/disk.c
index 9e008a3..da06601 100644
--- a/cmd/disk.c
+++ b/cmd/disk.c
@@ -4,6 +4,7 @@
  * Wolfgang Denk, DENX Software Engineering, wd@denx.de.
  */
 #include <common.h>
+#include <bootstage.h>
 #include <command.h>
 #include <cpu_func.h>
 #include <image.h>