Working in branch feature-twincpu, I did the following:
mkdir build_dep_check
pushd build_dep_check
cmake -G"Unix Makefiles" -DPROJ_CONFIG=readlink -f ../configs/ConfigRegressionIPC.cmake -DTARGET_PLATFORM=psoc6 -DCOMPILER=ARMCLANG -DCMAKE_BUILD_TYPE=Debug ../
popd
cmake --build build_dep_check -- -j VERBOSE=1
rm build_dep_check/tfm_sign.bin
cmake --build build_dep_check -- -j VERBOSE=1

tfm_sign.bin does not get re-built.

I suspect the same is true for tfm_full.bin.