Improved assert handling

[krichardsson]

Reset assert data after re-boot to make sure the CF starts cleanly after next reboot.
In most cases this is not a problem, but might cause some surprise when flashing using a ST-link after an assert.

1. CF hits a failing asser and re-boots
2. It starts up and logs assert data
3. The user fixes the problem and re-flashes the STM using a ST-link
4. The STM restarts but (in the current implementation) finds the old assert information again and logs assert info again.

This PR clears the assert info to make the STM start cleanly in step 4