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/Arcturus/ucp1020/cmd_arc.c b/board/Arcturus/ucp1020/cmd_arc.c
index 7a510c6..755f569 100644
--- a/board/Arcturus/ucp1020/cmd_arc.c
+++ b/board/Arcturus/ucp1020/cmd_arc.c
@@ -10,6 +10,7 @@
 
 #include <common.h>
 #include <div64.h>
+#include <env.h>
 #include <malloc.h>
 #include <spi_flash.h>
 #include <mmc.h>