The u-boot to be used depends on wether you will be using the FSP BSP Release or the FSL Community BSP. There is a bit more information on which layers are included on each on the following document:
The conf folder is located on the top of each layer, not every individual recipe, or you may use the conf folder on the Build directory for global changes.
I hope this helps!
I guess i am using the FSP BSP Release as i am using the download from L3.14.28_1.0.0_iMX6SL_BUNDLE. So for this case, i should be using the u-boot source from (2)?
The only change i am making is from the 1GB RAM to 512MB fyi. Hope you can advise, thanks!