env: Move env_set_hex() to env.h

Move env_set_hex() over to the new header file along with env_set_addr()
which uses it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/board/sunxi/board.c b/board/sunxi/board.c
index 98bc3cd..bb425cb 100644
--- a/board/sunxi/board.c
+++ b/board/sunxi/board.c
@@ -12,6 +12,7 @@
 
 #include <common.h>
 #include <dm.h>
+#include <env.h>
 #include <mmc.h>
 #include <axp_pmic.h>
 #include <generic-phy.h>