mbedtls does not build with -fPIC by default

[JohnHoder]

mbedtls does not build with -fPIC in LOCAL_CFLAGS causing the linking of proxenet to fail.
Not that you can do anything about it but it might be a good idea to make people aware of the issue and how to fix it.

[hugsy]

CC @ARMmbed

Good point, I will add a note in the documentation. Can you drop an issue on https://github.com/ARMmbed/mbedtls/issues ?

Thanks.