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/memory_buffer_alloc.c b/library/memory_buffer_alloc.c
index 51ea7c4..ca08e04 100644
--- a/library/memory_buffer_alloc.c
+++ b/library/memory_buffer_alloc.c
@@ -315,7 +315,7 @@
             mbedtls_exit( 1 );
 
         ret = (unsigned char *) cur + sizeof( memory_header );
-        memset( ret, 0, original_len );
+        mbedtls_platform_memset( ret, 0, original_len );
 
         return( ret );
     }
@@ -373,7 +373,7 @@
         mbedtls_exit( 1 );
 
     ret = (unsigned char *) cur + sizeof( memory_header );
-    memset( ret, 0, original_len );
+    mbedtls_platform_memset( ret, 0, original_len );
 
     return( ret );
 }
@@ -438,7 +438,7 @@
         if( hdr->next != NULL )
             hdr->next->prev = hdr;
 
-        memset( old, 0, sizeof(memory_header) );
+        mbedtls_platform_memset( old, 0, sizeof(memory_header) );
     }
 
     // Regroup with block after
@@ -477,7 +477,7 @@
         if( hdr->next != NULL )
             hdr->next->prev = hdr;
 
-        memset( old, 0, sizeof(memory_header) );
+        mbedtls_platform_memset( old, 0, sizeof(memory_header) );
     }
 
     // Prepend to free_list if we have not merged
@@ -572,7 +572,7 @@
 
 void mbedtls_memory_buffer_alloc_init( unsigned char *buf, size_t len )
 {
-    memset( &heap, 0, sizeof( buffer_alloc_ctx ) );
+    mbedtls_platform_memset( &heap, 0, sizeof( buffer_alloc_ctx ) );
 
 #if defined(MBEDTLS_THREADING_C)
     mbedtls_mutex_init( &heap.mutex );
@@ -593,7 +593,7 @@
              - (size_t)buf % MBEDTLS_MEMORY_ALIGN_MULTIPLE;
     }
 
-    memset( buf, 0, len );
+    mbedtls_platform_memset( buf, 0, len );
 
     heap.buf = buf;
     heap.len = len;