Export kernel defconfig through android-build

I'm writing up some notes on building Android from scratch and replacing the kernel in an Android build from one built locally, and I realized that's it a bit of a chore to extract the kernel config that got used. I thought, it may make sense to provide an way in android-build to control what gets used - which would make finding this information easier since if would one of the configs that gets passed to the build like TARGET_PRODUCT. Thoughts?