Salt entropy pool mixer with hw RTC date and time

[isaacvr]

Feature description

Supply device RTC (i.e. RTC_TR and RTC_DR BCD registers) date and time when salting entropy

Is your feature request related to a problem? Please describe.
Need salt sources

Additional context
RTC not implemented in libopencm3 for STM32F2 family.

This is a type 3 salt source.

Possible implementation

• Reimplement for f2 the RTC API for F1 in our local copy of libopencm3
• Initialize RTC for the device in firmware's setup.c using the previous API
  • consider e.g. this example as a reference
  • Random date values at startup would be fine
• Prepare for RTC periodic (auto) wakeup unit
• Read RTC raw values and mix with entropy pool mixer
• Convert RTC values to string and mix with entropy pool mixer

Notice that RCC clock source might already be initialized for SysTick timer