bootstage: Plumb in bootstage calls for basic operations

This inserts bootstage calls into tftp, usb start and bootm. We
could go further, but this is a reasonable start to illustrate
the concept.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/net/net.c b/net/net.c
index 045405b..c5acf8f 100644
--- a/net/net.c
+++ b/net/net.c
@@ -402,6 +402,7 @@
 		NetArpWaitTxPacketSize = 0;
 	}
 
+	bootstage_mark_name(BOOTSTAGE_ID_ETH_START, "eth_start");
 	eth_halt();
 	eth_set_current();
 	if (eth_init(bd) < 0) {