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/mtd/cfi_flash.c b/drivers/mtd/cfi_flash.c
index 6b97e14..6c22f07 100644
--- a/drivers/mtd/cfi_flash.c
+++ b/drivers/mtd/cfi_flash.c
@@ -19,6 +19,7 @@
 #include <common.h>
 #include <console.h>
 #include <dm.h>
+#include <env.h>
 #include <errno.h>
 #include <fdt_support.h>
 #include <asm/processor.h>