The shared `build-libs` directory needs to be available for
modules/packages that depend on the SDK before that package
is eventually compiled.
Packages are downloaded, patched, prepared before any module
is compiled. By adding the directory creation as a dependency
of `PKG_PREPARE` we make sure the rule is run before compilation
starts.
The vendor binary libraries of ESP-IDF are provided as a separate GIT repository. These libraries are defined as separate package for two reasons: 1. RIOT packages don't support to clone GIT repositories recursively; 2. ESP-IDF pulls a lot of other GIT repositories that are not needed when it is cloned recursively.