I actually had a thread where I was discussing an option about doing this.
There is an awesome link provided by a user on the forums named carriba for a script from adafruit that was designed to do this for Raspbian, that I have used now 5 times and 100% success rate. Cool thing is that /dev/sda1 may or may not always point to your hdd where your root fs is located. It uses GPT to make sure it will always know where root fs is located and you can obtain a copy here...
Only issue is that if you want your splash screen the way its supposed to boot, your have to edit cmdline.txt and make the proper adjustments manually
- Monery