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/cmd/ubi.c b/cmd/ubi.c
index c511a2f..ca5dc90 100644
--- a/cmd/ubi.c
+++ b/cmd/ubi.c
@@ -13,6 +13,7 @@
 
 #include <common.h>
 #include <command.h>
+#include <env.h>
 #include <exports.h>
 #include <memalign.h>
 #include <mtd.h>