common: Move serial functions out of common.h

These functions belong in serial.h so move them over.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/common/board_f.c b/common/board_f.c
index e3591cb..0104445 100644
--- a/common/board_f.c
+++ b/common/board_f.c
@@ -26,6 +26,7 @@
 #include <os.h>
 #include <post.h>
 #include <relocate.h>
+#include <serial.h>
 #ifdef CONFIG_SPL
 #include <spl.h>
 #endif