Set aside two key slots that are always available to write but not read

[jbdatko]

Allow users to store keys in two slots designate as write always, read never. This will essentially allow user programmable keys.

Suggested by chaincrusher on IRC.

[jbdatko]

Note: This feature is not compatible with the 0.1.0 version since the configuration settings have changed and should be targeted for the 1.0.0 release.

[jbdatko]

Latest attempt is commit 3708613. However, the write is not working. I've contact Atmel for help. My guess is that my tempkey hash doesn't match the device at the time of the write.

However, Atmel confirms that my strategy seems sound :)