In practice I have found that it would be very convenient to just specify the in-tree defconfig name in the kernel bitbake recipe instead of copying it from kernel source into own layer metadata manually every time it needs to change. Something like this is implemented in Phytec's custom kconfig class here: https://git.phytec.de/meta-phytec/tree/classes/kconfig.bbclass?h=dunfell The variable is INTREE_DEFCONFIG Also something similar is supposed to work with the kernel-yocto class and KBUILD_DEFCONFIG variable, though I have not been able to make it work properly for my mainline Linux 5.8.5 fork. Anyway, I don't think such a feature should be exclusive to kernel-yocto or linux-yocto, but should be available using plain kernel.bbclass
This isn't something we can or want to standardize in kernel.bbclass. There are literally hundreds of users, and use cases for the core kernel.bbclass. We can't keep them all happy, so we need to keep things minimal there. Note: there is an outstanding feature to move the fragment support into a core location and make it optional, at that point, this will be addressed. Closing this as "won't implmenent", since we already have something in bugzilla. As for KBUILD_DEFCONFIG, it works here on all kernel versions, so you should open a specific bug with reproducing the issue you are seeing. *** This bug has been marked as a duplicate of bug 8191 ***