MS5611 sends additional byte after NACK
The i2c_transfer7 function of libopencm3 is implemented using I2C with polling, but isn't following the STM32 Reference manual (RM0008, Page 759, Method 2), which states special handling of 1 and 2 byte receives. This results in clearing the ACK-bit at the wrong time.

See http://libopencm3.org/docs/latest/stm32f2/html/i2c__common__v1_8c_source.html#l00492