while developing esp-camera I had the problem that firmware.elf was
not regenerated if the only file I edited was in the esp32-camera
submodule.
By telling make that $(IDF_CMAKE_TARGETS), the list of .a fliles
produced by ninja-building esp-idf, depend on esp-idf-stamp and that
firmware.elf in turn depends on these files, the ninja build is re-run
on each make, which is reasonably quick if nothing needs to be rebuilt.
2b0f1cd087