net: Fix net buffer initialization

A new non-static function net_init() will initialize buffers and
read from the environment.  Only update from the env on each entry
to NetLoop().

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Simon Glass <sjg@chromium.org>

include/net.h

@@ -436,6 +436,7 @@ extern IPaddr_t Mcast_addr;
 #endif
 
 /* Initialize the network adapter */
+extern void net_init(void);
 extern int NetLoop(enum proto_t);
 
 /* Shutdown adapters and cleanup */