Replace memset() with mbedtls_platform_memset()

Steps:

1. sed -i 's/\bmemset(\([^)]\)/mbedtls_platform_memset(\1/g' library/*.c tinycrypt/*.c include/mbedtls/*.h scripts/data_files/*.fmt

2. Manually edit library/platform_util.c to revert to memset() in the
implementations of mbedtls_platform_memset() and mbedtls_platform_memcpy()

3. egrep -n '\<memset\>' library/*.c include/mbedtls/*.h tinycrypt/*.c
The remaining occurrences are in three categories:
    a. From point 2 above.
    b. In comments.
    c. In the initialisation of memset_func, to be changed in a future commit.
diff --git a/library/ecdh.c b/library/ecdh.c
index c572687..03763ff 100644
--- a/library/ecdh.c
+++ b/library/ecdh.c
@@ -179,7 +179,7 @@
     mbedtls_ecp_point_init( &ctx->Vf  );
     mbedtls_mpi_init( &ctx->_d );
 #else
-    memset( ctx, 0, sizeof( mbedtls_ecdh_context ) );
+    mbedtls_platform_memset( ctx, 0, sizeof( mbedtls_ecdh_context ) );
 
     ctx->var = MBEDTLS_ECDH_VARIANT_NONE;
 #endif