Fix incorrect checksum calculation for paged erase

Based on jsnyder#4 .

Author: florisla

stm32loader.py

@@ -307,7 +307,7 @@ def _page_erase(self, pages):
         # page erase, see ST AN3155
         nr_of_pages = (len(pages) - 1) & 0xFF
         self.serial.write(bytes([nr_of_pages]))
-        checksum = 0xFF
+        checksum = nr_of_pages
         for page_number in pages:
             self.serial.write(bytes([page_number]))
             checksum = checksum ^ page_number