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/net/net.c b/net/net.c
index 58b0417..1a175c4 100644
--- a/net/net.c
+++ b/net/net.c
@@ -90,6 +90,7 @@
 #include <common.h>
 #include <command.h>
 #include <console.h>
+#include <env.h>
 #include <environment.h>
 #include <errno.h>
 #include <net.h>