test: Add an MbedTLS vs TinyCrypt comparison on secp256r1

Add a simple test to compare the footprint of MbedTLS vs
TinyCrypt for the secp256r1 curve. On the MbedTLS side
the test uses the P256M driver developed by the MbedTLS
team. This is an optimized implementation which targets
the secp256r1 curve.

The example itself is pretty easy:
- generate 2 EC keys
- perform key agreement

Signed-off-by: Valerio Setti <vsetti@baylibre.com>