Add initfs hook which causes a kernel panic (useful for debugging)

This hook is useful (at least for me) to debug the mainline kernel when there's no serial output from the device.

The hook simply causes a kernel panic after 30 seconds, and these are the reasons:

1. If the device remains powered on for 30s before rebooting I know that he reached the initramfs script otherwise it failed earlier
2. Installing a working kernel with CONFIG_RAMOOPS=y and the debug-shell hook in the boot partition, after the kernel panic it will reboot and you can telnet in and read /proc/last_kmsg