On a valid RSA public key, mbedtls_rsa_export should succeed if you
ask for the public fields, but fail if you ask for private fields. The
code was expecting to succeed when asking for private fields, so
failed on every valid RSA public key.
git grep -Fl /config.pl | xargs sed -i -e 's!/config\.pl!/config.py!g'
Also:
* Change one comment in include/mbedtls/check_config.h.
* Change PERL to PYTHON in CMakeLists.txt.
If we try to delete a non-existant file using del on Windows, as
can happen when running make clean, del will throw an error. Make
the Makefiles more robust by only deleting files if they exist.
