- boot the I2C after init in low power mode
- otherwise I2C will consume more power until the first time it is
used, which is surprising
- STM32 F1 only: reconfigure SCL and SDA as GPIOs while the I2C
peripheral is powered down
- When the I2C peripheral is not clocked, it drives SCL and SDA
down. This will dissipate power across the pull up resistor.