Improve EKF alt-hold with VL53 sensor

[ataffanel]

When flying with the VL53 time of flight sensor facing down and the EKF (see #174), the EKF is trying to correct roll and pitch even though there is not information available for X and Y.

Another problem is that the velocity output of the EKF has a drift which makes the Crazyflie slowly go down.

The first problem could be solved by decoupling X,Y,PX and PY states from the EKF like so:

# Pseudo code by Mike Hamer
for i in [STATE_X, STATE_PX, STATE_Y and STATE_PY]:
  # set all covariances 0
  for j in ALL_STATES:
    S[i][j] = 0

  # set state variance maximum
  P[i][i] = MAX_COVARIANCE
  # set state zero
  X[i] = 0

[ataffanel]

The commit 2b0958d implements the pseudo-code from @mikehamer and it works fine. It is still implemented as a compile flag and might need to be done automatically in the future (like disabling X/Y/Z by default and enabling them only if some meaningful sensor update enters the EKF).