cpu/atmega_common: Move stdio_init() to cpu_init()

Moving atmega_stdio_init() to cpu_init() just before periph_init() guarantees
that stdio is available to allow DEBUG() in periph_init(). This also helps to
unify the boot up process of ATmega boards and de-duplicates the stdio init from
board_init().

boards/common/atmega/board.c

@@ -34,10 +34,6 @@ void board_init(void)
 
     atmega_set_prescaler(CPU_ATMEGA_CLK_SCALE_INIT);
 
-#ifdef MODULE_AVR_LIBC_EXTRA
-    atmega_stdio_init();
-#endif
-
     cpu_init();
     led_init();
     irq_enable();

boards/waspmote-pro/board.c

@@ -58,10 +58,6 @@ void board_init(void)
 #endif
 #endif
 
-#ifdef MODULE_AVR_LIBC_EXTRA
-    atmega_stdio_init();
-#endif
-
     cpu_init();
     led_init();
     irq_enable();

cpu/atmega_common/cpu.c

@@ -105,6 +105,10 @@ void cpu_init(void)
     wdt_reset();   /* should not be nececessary as done in bootloader */
     wdt_disable(); /* but when used without bootloader this is needed */
 
+    /* Initialize stdio before periph_init() to allow use of DEBUG() there */
+#ifdef MODULE_AVR_LIBC_EXTRA
+    atmega_stdio_init();
+#endif
     /* Initialize peripherals for which modules are included in the makefile.*/
     /* spi_init */
     /* rtc_init */