env: Move env_get_f() to env.h

Move this function over to the new header file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/board/xes/common/board.c b/board/xes/common/board.c
index 66467c6..43575bc 100644
--- a/board/xes/common/board.c
+++ b/board/xes/common/board.c
@@ -4,6 +4,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include "fsl_8xxx_misc.h"
 
 int checkboard(void)