The folder hosts mbedtls third party library. The source code is in src folder. A build script BUILD.gn is provided and defines a library target for baremtal use. A number of features are disabled. See config_baremetal_exp.h for more detail. To use the target, add //third_party/mbedtls:mbedtls_lib_baremetal to the dependency list.
sources.gni contains the list of source files for building. It is generated by generate_sources_gni.py based on current version of the source code. If source version bumps up, it may be necessary to re-run the script to keep the file in sync. The longer term plan is to set up auto-roll and update the file as part of the rolling process.