Merge branch 'feature/newlib_locking' into 'master'

newlib locking support

Has matching tests here: http://gitlab.espressif.cn/idf/esp-idf-tests/commits/feature/newlib_locking_tests

This is ready, the "bug" I saw was me not understanding how newlib stream locking works (I still don't understand why it works like it does, but that's less important for now.)

See merge request !17