When specifying CONFIG_KERNEL_GIT_CLONE_URI a clean build fails, as the system assumes a already downloaded/cloned/packed linux-source is present in dl/ which on a freshly checked out openwrt/lede source isn't the case yet, when toolchain/kernel-headers is about to be built.
The build error looks like:
make: Entering directory `/build/lede.git/toolchain/kernel-headers'
zcat /build/lede.git/dl/linux-4.4.42.tar.gz | tar -C /build/lede.git/build_dir/toolchain-arm_cortex-a9+neon_gcc-6.3.0_glibc-2.24_eabi -xf -
gzip: /build/lede.git/dl/linux-4.4.42.tar.gz: No such file or directory
tar: This does not look like a tar archive
tar: Exiting with failure status due to previous errors
The issue apparently is located in include/kernel-defaults.mk, line 53ff::