Setting Persistence doesn't work

[matthewSorensen]

Alterations don't survive a power cycle, even though flexram code works in isolation and a read right after the write reports a valid checksum.

[matthewSorensen]

Settings persistence itself works - if I disable checking the checksum, parameters survive power cycles, and don't seem to get corrupted. I may be configuring the checksum hardware wrong - if not, and switching to a software one works, this should fix the problem.

[matthewSorensen]

Commit d228262 switches the checksum over to the original grbl 8-bit sum. Recovering stored settings now works, although I haven't verified if work offsets or start-up lines work.