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/common/bootm_os.c b/common/bootm_os.c
index 1d58462..4402353 100644
--- a/common/bootm_os.c
+++ b/common/bootm_os.c
@@ -6,6 +6,7 @@
 
 #include <common.h>
 #include <bootm.h>
+#include <bootstage.h>
 #include <cpu_func.h>
 #include <efi_loader.h>
 #include <env.h>