Add key test mode when no SD card present

[Speccery]

When bringing up hardware, it would be advantageous to be able to test hardware functionality piece by piece rather than requiring many things to work (SD, keys, display) at once. One simple way for this would be to detect SD card, and if not present, enter a key test mode which would enable testing all keys. This way only display and keys would need to work to test keys.

Current behaviour: if no SD card, no message is shown. It is possible to press left and right to switch between Load and New, but impossible to know if other keys work (I'm thinking of you "A" key).

[maks]

This is a great idea and I for one would find this useful when building devices.