run: capture kernel msg to detect boot errors

Handle boot errors like the old run tool did by checking a kernel
specific string on boot up and start the run script timeout afterwards.

Issue #1395.

tool/run/boot_dir/fiasco

@@ -59,6 +59,11 @@ proc bin_dir { } {
 set fiasco_serial_esc_arg "-serial_esc "
 
 
+proc run_boot_string { } {
+	return "L4 Bootstrapper"
+}
+
+
 ##
 # Populate boot directory with binaries on fiasco
 #