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>

board/liebherr/display5/display5.c

@@ -13,6 +13,7 @@
 #include <asm/arch/mx6-pins.h>
 #include <asm/arch/mx6-ddr.h>
 #include <asm/arch/sys_proto.h>
+#include <env.h>
 #include <errno.h>
 #include <asm/gpio.h>
 #include <malloc.h>