source for a working booloader

[sansari123]

Does anyone know how / where I can get the source for a bootloader that works with msm8974? I am talking about the loader that comes with the stock image. I believe that is sbl2.mbn? Also what is an mbn file and what tool or application do I use to unpack it? I do realize that for normal flashing, I don't need the source or that I need to unpack it. I am trying to see how I can add my own modifications.