altoslib: Compute tilt angle from eeprom data

This copies the computation of tilt angle from the firmware so that
post-flight analysis can also show the data.

This change also renames all of the imu values to make them easier to
understand:

	accel	gyro	axis

	along	roll	length of the board
	across	pitch	across the board
	through	yaw	through the board.

Signed-off-by: Keith Packard <keithp@keithp.com>