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/tinycrypt/ecc_dh.c b/tinycrypt/ecc_dh.c
index ec1328e..54b9a8a 100644
--- a/tinycrypt/ecc_dh.c
+++ b/tinycrypt/ecc_dh.c
@@ -105,7 +105,7 @@
 				       _public + curve->num_words);
 
 		/* erasing temporary buffer used to store secret: */
-		memset(_private, 0, NUM_ECC_BYTES);
+		mbedtls_platform_memset(_private, 0, NUM_ECC_BYTES);
 
 		return 1;
 	}
@@ -146,7 +146,7 @@
 					       _public + curve->num_words);
 
 			/* erasing temporary buffer that stored secret: */
-			memset(_private, 0, NUM_ECC_BYTES);
+			mbedtls_platform_memset(_private, 0, NUM_ECC_BYTES);
 
       			return 1;
     		}