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/dhelectronics/dh_imx6/dh_imx6.c b/board/dhelectronics/dh_imx6/dh_imx6.c
index 40cc2a8..1994dec 100644
--- a/board/dhelectronics/dh_imx6/dh_imx6.c
+++ b/board/dhelectronics/dh_imx6/dh_imx6.c
@@ -21,6 +21,7 @@
 #include <asm/mach-imx/sata.h>
 #include <ahci.h>
 #include <dwc_ahsata.h>
+#include <env.h>
 #include <environment.h>
 #include <errno.h>
 #include <fsl_esdhc_imx.h>