i2c: add asynchronous write
i2c_read() and i2c_write() are blocking functions, and their comments
have been updated to reflect this.
The new i2c_write_async() function is starting an i2c transfer, and
returns immediatly. All i2c transfer functions will now return -EAGAIN
if there is already a transfer in progress (-EBUSY was conflicting with
arbitration lost).
ssd130x_display_full_screen() has been updated to use this new
asynchronous write to start the transfer of the full screen. All
i2c_write* calls are placed in a retry loop if the call returns -EAGAIN.
