example of i2c slave device supporting block transfers

Dependencies:   lib_i2c_slave_block

Tested on nucleo board as slave. Use with i2c_master_block_example on raspberry pi as master.

Master I2C write will print on serial terminal 115200. User button will raise an irq; the RPi test program will service the interrupt.

See lib_i2c_slave_block for explanation of code.