netboot: Cope with u-boot warning string

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/netboot/netboot b/netboot/netboot
index 3c941a0..da289bf 100755
--- a/netboot/netboot
+++ b/netboot/netboot
@@ -72,9 +72,10 @@
                 i, m, d = conn.expect_or_raise([
                     r"Bytes transferred = \d+ \([0-9a-f]+ hex\)",
                     "Not retrying...",
+                    r"## Warning:",
                     r"[*]{3} ERROR:",
                 ])
-                if i == 1 or i == 2:
+                if i > 0:
                     print("Error detected, exiting", file=sys.stderr)
                     return