Force CPU-hang on errors in FW

[secworks]

I suggest that errors caught by the FW should be handled by forcing the CPU to hang, not by jumping to an error loop that does the LED blinking. A SW loop could possibly be forced to jump somewhere. By triggering an illegal instruction, the CPU will hang and stop executing. The external effect will be the same - blinking red.

[dehanj]

Use assert_fail() from tkey-libs.
This will harmonize code used and lower the amount of code to maintain.

[mchack-work]

See #129

[dehanj]

Closes with #168