i2c: atmega: implemented master xfer start

atmega_i2c_xfer:
	This function prepares the bus to start an I2C master
	transmission. It sets up all the (file) global variables which a
	typical master-to-slave transmission needs. As soon as the I2C START
	bit has been generated on the bus, the executing thread will be
	placed on a waiting queue, if the scheduler is enabled, untill the
	entire transmission is done. If the scheduler is not enabled,
	atmega_i2c_xfer will do NOP's untill the transmission has completed.

Signed-off-by: Michel Megens <dev@michelmegens.net>