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/drivers/net/fm/fm.c b/drivers/net/fm/fm.c
index 0a43dfe..ce0f167 100644
--- a/drivers/net/fm/fm.c
+++ b/drivers/net/fm/fm.c
@@ -4,6 +4,7 @@
  *	Dave Liu <daveliu@freescale.com>
  */
 #include <common.h>
+#include <env.h>
 #include <malloc.h>
 #include <asm/io.h>
 #include <linux/errno.h>