StandbyRTCMode_Measure() function

This function configures the system to enter Standby mode with RTC clocked by LSE or LSI for current consumption measurement purpose. STANDBY Mode with RTC clocked by LSE/LSI ======================================== - RTC Clocked by LSE/LSI - IWDG OFF - Backup SRAM OFF - Automatic Wakeup using RTC clocked by LSE/LSI (after ~20s)