Integrate p256-m as an example driver alongside Mbed TLS and write documentation for the example.

(Reapplying changes as one commit on top of development post codestyle change instead of rewriting old branch)

Signed-off-by: Aditya Deshpande <aditya.deshpande@arm.com>
diff --git a/scripts/config.py b/scripts/config.py
index ac5f77c..92a4aad 100755
--- a/scripts/config.py
+++ b/scripts/config.py
@@ -216,6 +216,7 @@
     'MBEDTLS_TEST_CONSTANT_FLOW_VALGRIND', # build dependency (valgrind headers)
     'MBEDTLS_X509_REMOVE_INFO', # removes a feature
     'MBEDTLS_SSL_RECORD_SIZE_LIMIT', # in development, currently breaks other tests
+    'MBEDTLS_P256M_EXAMPLE_DRIVER_ENABLED' # influences SECP256R1 KeyGen/ECDH/ECDSA
 ])
 
 def is_seamless_alt(name):