sensorsmonitor.py: Flush stdout after every batch.

This is convenient when piping through other tools like `grep` or `tee`.
By default when stdout is not directed to a terminal it becomes
block-buffered, so output is delayed until the output buffer is
completely full.
Convert to SI units inside SMACCMPilot.Hardware drivers.

Note that the values coming from the hardware are signed, in
two's-complement representation, but were being processed in the
firmware as if they were unsigned. The Python script then re-interpreted
the telemetry bytestream as signed numbers, which worked fine until I
tried to do any kind of math on the firmware's representation.
ivory-px4-tests: Tell git to ignore build products.