Use custom panic handler for panic-persist

[dbrgn]

Sleep for a while until restarting to avoid draining the battery with a panic loop.

This can be done using a custom panic handler:

https://github.com/jamesmunns/panic-persist#custom-panic-handler

• Persist panic info
• Set RTC wakeup timer
• Go to standby mode

[dbrgn]

Putting this on hold, see https://github.com/gfroerli/firmware/pull/99#issuecomment-883708941