Semaphore to syncronize access to the I2C interface

[Z4nna]

In the barometer and the imu nodes, add a named semaphore to prevent race conditions accessing the iic interface. In particular:

1. Create the semaphore with the starting value of 1.
2. The first node to run first should call sem_wait() and then init the iic interface.
3. After the initialization is done call sem_post() .
4. Whenever one node needs to access the iic interface ( critical section of the code ), it should call sem_wait() and then, after it’s done, give it back with sem_post().