Please, clone the kernel source code from linux kernel repository (such as linux-imx.git - i.MX Linux Kernel ) checkout the proper branch and develop (or change) accordingly with your needs outside the yocto structure.
When you have the patches or the complete kernel, you can reintegrate it to the yocto, so you have the automatic linux distribution tool to work with.
The tmp folder is temporary, it sounds obvious and it is, but a lot of people really like to develop using this folder, however it's not recommended for a long term development cycle.
In order to understand a little bit more about how to work with yocto project with different roles you can take a look here:
Amazon.com: Heading for the Yocto Project eBook: Otavio Salvador, Daiane Angolini: Kindle Store
if the book is not free today for you, you can take this also from:
GitHub - CollaborativeWritersHub/heading-for-the-yocto-project
and PDF:
https://github.com/CollaborativeWritersHub/heading-for-the-yocto-project/releases/download/v0.1/Head...
as it's an open sourced book.
