Dwgdbserver

- Fixed: Before starting the final ISP session in order to unprogram the DWEN fuse, another restart of the tool is performed by housekeeping end_session/start_session. This fixes the problem I observed earlier when the fusebit was not unprogrammed after leaving the debugger.
- Added: In dw-harvest.py, the `OCD` property `BUFFERS_PER_FLASH_PAGE` is evaluated. This seems to be the one that signals that the MCU has a 4-page erase command. I yet have to find a way of not deleting the other 3 pages when writing to one of the pages in a block of 4 pages.

- Fixed: The packet parsing procedure had ignored the fact that
single-letter commands can be immediately followed by a hex number;
now we treat those commands in the right way.

- Added: `monitor noload` will allow execution even without a previous
load command.

- Changed: All debugWIRE relevant start/finish methods have been moved
to dw-gdbserver. The code has been placed into a new class DebugWIRE.

- Fixed: After enabling debugWIRE by setting the DWEN fuse and power-cycling, the
debugging tool is restarted by a
housekeeper.end\_session()/housekeeper.start\_session() pair.
With that, debugging works now after having just enabled the debugWIRE mode.

- Much of the startup/shutdown code is now modularized and ready to be
moved over to dw-gdbserver