common: Move functions for loading from fat/ext2 to fs.h

These are filesystem functions and belong in the filesystem header file.
Move them.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/pxe.c b/cmd/pxe.c
index 194a366..bce6728 100644
--- a/cmd/pxe.c
+++ b/cmd/pxe.c
@@ -6,6 +6,7 @@
 
 #include <common.h>
 #include <command.h>
+#include <fs.h>
 #include <net.h>
 
 #include "pxe_utils.h"