Define placeholder fastboot serial number value

- For now, set the fastboot serial to 0123456789ABCDEF, as LK did
previously. Change this in EVT2 timeframe when we have the crypto chip
onboard.

Change-Id: I2f062f60031eb42cbc127fda14a58b71b077707d
diff --git a/board/mediatek/mt8516-coral/mt8516-coral.c b/board/mediatek/mt8516-coral/mt8516-coral.c
index 4299553..2f17a88 100644
--- a/board/mediatek/mt8516-coral/mt8516-coral.c
+++ b/board/mediatek/mt8516-coral/mt8516-coral.c
@@ -51,6 +51,9 @@
 			}
 		}
 	}
+	
+	env_set("serial#", "0123456789ABCDEF");
+
 	return 0;
 }