Need to run git submodule sync when modifying/upgrading/downgrading an existing copy of IDF

[igrr]

IDF project will change some of the submodule URLs in .gitmodules file. Because of this, git submodule update may fail if the old submodule URL doesn't contain the new commit (and vice versa). To mitigate this, we should also sync submodule URLs after doing git checkout, using git submodule sync command.