Read from and write to MKM when panic button is pressed.

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+Tamper detection for Atmel AVR ATtiny828 MCU on the Cryptech alpha
+board, rev02.
+
+       *************
+       * P A N I C *
+       *   button  *
+       *************
+        /
+       /
+      /
+    AVR ---- SPI mux ---- FPGA
+                |          |
+                |         ARM
+               MKM
+
+AVR -- Atmel MCU
+FPGA -- FPGA
+MKM -- Master Key Memory, 23K640 SRAM
+SPI mux -- 2 x MC74AC244DW
+ARM -- ARM CPU
+
+The MKM holds the master key for the device.
+
+The AVR, MKM and the mux are battery powered.
+
+The AVR and the FPGA are sharing access to the MKM through the mux,
+with the AVR connected to the pins used for deciding who's in control
+of the memory. The FPGA is in control by default.
+
+When the panic button is pressed, the AVR grabs the MKM and writes
+zeros to it as quickly as possible. In idle mode, i.e. when the panic
+button is not pressed, the AVR tries to consume as little power as
+possible.