Add preliminary support for some STM32G0 chips. (closes #759) (#760)

* Add preliminary support for some STM32G0 chips.

* Fix a bug in the 'which page to erase' logic.

When I flashed a program with more than one page, I noticed that this logic did not un-set previously-set 'PNG' bits. So it would erase page 1, then page 3, then page 3...