We potentially save flash space by not enabling Mbed TLS self-tests
by default. A new test config file, TESTS/configs/mbedtls.json, is
provided to enable self tests. This newly created JSON file also
enables timing in Mbed TLS so timing gets tested.
The Mbed TLS importer accidentally imports Makefile and .gitignore
from Mbed TLS's `library/` directory. This commit restricts the
pattern to .h and .c files only, and removes the unnecessary files.
Files have been automatically imported by setting MBED_TLS_RELEASE to
mbedtls-2.24.0 in connectivity/mbedtls/tools/importer/Makefile and
running `make` in that directory.