Did you install all the feeds? Usually I only link the packages I want to modify from the feeds subdirs to the packages subdir. Seen your issue at the git repo. Can be you just need ccache.
If you modify the kernel getting the full build environment is nice. You can try this first
git clone -b test https://github.com/CZ-NIC/turris-os
cd turris-os/
./compile_omina_fw
This should build everything.
Afterwards you can customize it using make menuconfig etc.