under source folder you will *NOT* find any package source code.
That source is the recipe source that generates the package.
For example, the kernel.
The source code you find under fsl-community-bsp/source/meta-fsl-arm/recipe-kernel/linux/* is the yocto recipe for the kernel. The build/install/deploy recipe for the kernel.
So, if you want to develop using kernel as base, you must have a copy of this kernel from somewhere else. If you take a look at kernel recipe (for example meta-fsl-arm - Layer containing Freescale ARM hardware support metadata) it will download only SRCREV = "328597018dc58c4ffa38461db09e45bef62af227", it means a git commit hash.
So, if you create one commit over that hash, the default kernel recipe will *not* take your commit.
You can find the package tarball/git under fsl-community-bsp/downloads.
But, for kernel and u-boot I highly recommend you to have a separated folder for each kernel and u-boot that you will use during your development.
When you think you're done with development (or want to release a alpha/beta) you create your linux recipe, on your meta layer.
To build the kernel during your development cycle, you may use meta-toolchain.
you find a how to build kernel manually, a how to create you meta layer and other things on my Yocto Training.
The yocto source code is only "script" files that automatize the fetch/patch/configure/build/deploy/install process. The package source code itself is not under yocto source folder.