env: Move env_set() to env.h

Move env_set() over to the new header file.

Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/freescale/mx6ullevk/mx6ullevk.c b/board/freescale/mx6ullevk/mx6ullevk.c
index 1f0f70e..e119347 100644
--- a/board/freescale/mx6ullevk/mx6ullevk.c
+++ b/board/freescale/mx6ullevk/mx6ullevk.c
@@ -14,6 +14,7 @@
 #include <asm/mach-imx/boot_mode.h>
 #include <asm/io.h>
 #include <common.h>
+#include <env.h>
 #include <fsl_esdhc_imx.h>
 #include <linux/sizes.h>
 #include <mmc.h>