I2C: wrap the currently abandoned code for reading properly.

Not removed entirely, because it might be needed later.

i2c.c

@@ -345,8 +345,9 @@ ISR(TWI_vect) {
       // Try to resend when the bus became free.
       TWCR = (1<<TWINT)|(I2C_MODE<<TWEA)|(1<<TWSTA)|(0<<TWSTO)|(1<<TWEN)|(1<<TWIE);
       break;
-#if 0
-// Note: we currently have no receive buffer, so we can't receive anything.
+
+    #ifdef I2C_READ_SUPPORT
+
     case TW_MR_SLA_ACK:
       // SLA+R was sent, got АСК, then received a byte.
       if (i2c_index + 1 == i2c_byte_count) {
@@ -381,7 +382,8 @@ ISR(TWI_vect) {
       // Send stop condition.
       TWCR = (1<<TWINT)|(I2C_MODE<<TWEA)|(0<<TWSTA)|(1<<TWSTO)|(1<<TWEN)|(0<<TWIE);
       break;
-#endif
+
+    #endif /* I2C_READ_SUPPORT */
 
     #ifdef I2C_SLAVE_MODE
 

i2c.h

@@ -22,6 +22,19 @@
 */
 //#define I2C_SLAVE_MODE
 
+/** \def I2C_READ_SUPPORT
+
+  Currently, reading from the I2C bus is implemented only partially. The
+  already existing parts are wrapped with this #define.
+
+  If reading from I2C is needed, at least a read buffer has to be added. This
+  buffer used to be 'i2c_buffer[]', a pointer 'i2c_index' into this buffer and
+  a variable 'i2c_byte_count' to track on how many bytes should be read.
+  Further requirements are adjustments to the wrapped code to use this buffer
+  and a function to make the read data available to calling code.
+*/
+//#define I2C_READ_SUPPORT
+
 /** \def I2C_EEPROM_SUPPORT
 
   It's currently unclear what this enables exactly, apparently something to