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/nand.c b/cmd/nand.c
index 899d504..27efef2 100644
--- a/cmd/nand.c
+++ b/cmd/nand.c
@@ -23,6 +23,7 @@
 #include <linux/mtd/mtd.h>
 #include <command.h>
 #include <console.h>
+#include <env.h>
 #include <watchdog.h>
 #include <malloc.h>
 #include <asm/byteorder.h>