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/pkparse.c b/library/pkparse.c
index 4cff8d7..cea5a7f 100644
--- a/library/pkparse.c
+++ b/library/pkparse.c
@@ -654,7 +654,7 @@
     int ret;
     mbedtls_asn1_buf alg_oid;
 
-    memset( params, 0, sizeof(mbedtls_asn1_buf) );
+    mbedtls_platform_memset( params, 0, sizeof(mbedtls_asn1_buf) );
 
     if( ( ret = mbedtls_asn1_get_alg( p, end, &alg_oid, params ) ) != 0 )
         return( MBEDTLS_ERR_PK_INVALID_ALG + ret );