The shared `build-libs` directory needs to be available for
modules/packages that depend on the SDK before that package
is eventually compiled.
This also includes header files such as `esp32_idf_version.h`.
Packages are downloaded, patched, prepared before any module
is compiled. By adding the directory creation and header as
a dependency of `PKG_PREPARE` we make sure the rules are ran
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.