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/x509_csr.c b/library/x509_csr.c
index 9b58a86..4407742 100644
--- a/library/x509_csr.c
+++ b/library/x509_csr.c
@@ -96,7 +96,7 @@
     unsigned char *p, *end;
     mbedtls_x509_buf sig_params;
 
-    memset( &sig_params, 0, sizeof( mbedtls_x509_buf ) );
+    mbedtls_platform_memset( &sig_params, 0, sizeof( mbedtls_x509_buf ) );
 
     /*
      * Check for valid input
@@ -383,7 +383,7 @@
  */
 void mbedtls_x509_csr_init( mbedtls_x509_csr *csr )
 {
-    memset( csr, 0, sizeof(mbedtls_x509_csr) );
+    mbedtls_platform_memset( csr, 0, sizeof(mbedtls_x509_csr) );
 }
 
 /*