I have seen this warning a few times over the last years in the "make download" phase:
can't open file '/builder/shared-workdir/build/build_dir/target-x86_64_musl/linux-x86_64/linux-5.4.33/.config' at /builder/shared-workdir/build/scripts/kconfig.pl line 32.
That warning was generated by OpenWrt's on build robot. You will see it here:
The problem is that file .config does not exist yet when downloading. I guess the kernel configuration file will be created later on in the build process, if the right makefile target is requested.
I do not understand yet what STAMP_BUILT is about. In any case, it seems strange that the OpenWrt makefile will always run kconfig.pl , even if not needed for the specified makefile target.
I wonder what negative effects could have the fact that STAMP_BUILT has a different value the next time the same makefile is run and the kernel .config file actually exists.
Bulidroot is the OpenWrt term for the makefiles and scripts making up the build system. Iirc it was forked from or at least influenced by buildroot.net ages ago but nowadays it is something completely different.
Not sure which semicolon you're referring to, I haven't looked at the code in detail.