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/drivers/ddr/fsl/options.c b/drivers/ddr/fsl/options.c
index 4573ffa..b917931 100644
--- a/drivers/ddr/fsl/options.c
+++ b/drivers/ddr/fsl/options.c
@@ -5,6 +5,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <hwconfig.h>
 #include <fsl_ddr_sdram.h>