I2C: initial support.

This commit squashes in some fixes found after debugging on the
topic branch. Test code by Traumflug, collected from Ruslan's
code on the topic branch.

Before, same as now without I2C:

    FLASH  : 22408 bytes
    RAM    :  1393 bytes
    EEPROM :    32 bytes

Now with I2C:

    FLASH  : 23224 bytes
    RAM    :  2057 bytes
    EEPROM :    32 bytes

This totals to some 800 bytes with a whole lot of test code, so
implementation is pretty small :-)

cpu-avr.c

@@ -18,16 +18,24 @@
 */
 void cpu_init() {
   #ifdef PRR
-    #if defined SPI
-      PRR = MASK(PRTWI) | MASK(PRADC);
+    #if defined I2C && defined SPI
+      PRR = MASK(PRADC);
+    #elif defined SPI
+      PRR = MASK(PRADC) | MASK(PRTWI);
+    #elif defined I2C
+      PRR = MASK(PRADC) | MASK(PRSPI);
     #else
-      PRR = MASK(PRTWI) | MASK(PRADC) | MASK(PRSPI);
+      PRR = MASK(PRADC) | MASK(PRTWI) | MASK(PRSPI);
     #endif
   #elif defined PRR0
-    #if defined SPI
-      PRR0 = MASK(PRTWI) | MASK(PRADC);
+    #if defined I2C && defined SPI
+      PRR0 = MASK(PRADC);
+    #elif defined SPI
+      PRR0 = MASK(PRADC) | MASK(PRTWI);
+    #elif defined I2C
+      PRR0 = MASK(PRADC) | MASK(PRSPI);
     #else
-      PRR0 = MASK(PRTWI) | MASK(PRADC) | MASK(PRSPI);
+      PRR0 = MASK(PRADC) | MASK(PRTWI) | MASK(PRSPI);
     #endif
     #if defined(PRUSART3)
       // Don't use USART2 or USART3. Leave USART1 for GEN3 and derivatives.