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/foc

@@ -240,6 +240,11 @@ proc run_boot_dir_arm {binaries} {
 }
 
 
+proc run_boot_string { } {
+	return "L4 Bootstrapper"
+}
+
+
 ##
 # Populate boot directory with binaries on fiasco.OC
 #