A way to gain access to the flash content, would be for an attacker to reflash only a small part of the Arduino sketch. The code in this pull request tries to prevent this for a client issuing Write word, Write halfword or Write byte commands writing an Erase Row command into the CTRLA register. If these command writes are detected while Security Bit is set, the entire Arduino sketch is erased. Since Erase Row is required before a Flash Write may be executed, this should improve security.

The code changes still allows for sketch updates to be flashed using Arduino Studio and BOSSAC.