Switch to read-write-modify flashing.

This enables programming the chip without having to erase the entire contents.
Instead, JLink will erase where needed while retaining the data that's within
the same sectors but outside of the specified write area.

Closes square#154, square#148

pylink/jlink.py

@@ -2192,9 +2192,18 @@ def flash(self, data, addr, on_progress=None, power_on=False, flags=0):
         except errors.JLinkException:
             pass
 
-        res = self.flash_write(addr, data, flags=flags)
+        # Perform read-modify-write operation.
+        res = self._dll.JLINKARM_BeginDownload(flags=flags)
+        if res < 0:
+          raise errors.JLinkEraseException(res)
 
-        return res
+        bytes_flashed = self._dll.JLINKARM_WriteMem(addr, len(data), data)
+
+        res = self._dll.JLINKARM_EndDownload()
+        if res < 0:
+            raise errors.JLinkEraseException(res)
+
+        return bytes_flashed
 
     @connection_required
     def flash_file(self, path, addr, on_progress=None, power_on=False):

pylink/library.py

@@ -80,7 +80,10 @@ class Library(object):
         'JLINK_SWD_StoreRaw',
         'JLINK_SWD_SyncBits',
         'JLINK_SWD_SyncBytes',
-        'JLINK_SetFlashProgProgressCallback'
+        'JLINK_SetFlashProgProgressCallback',
+        'JLINKARM_BeginDownload',
+        'JLINKARM_EndDownload',
+        'JLINKARM_WriteMem'
     ]
 
     # On Linux and macOS, represents the library file name prefix