cpu/atmega_common: move clock init to common code

This code should not be in the realm of the board config, but in
common arch code.

boards/arduino-leonardo/board.c

@@ -20,20 +20,11 @@
 
 #include "board.h"
 #include "cpu.h"
-#include "periph/gpio.h"
-
-#ifndef CPU_ATMEGA_CLK_SCALE_INIT
-#define CPU_ATMEGA_CLK_SCALE_INIT    CPU_ATMEGA_CLK_SCALE_DIV1
-#endif
 
 void led_init(void);
 
 void board_init(void)
 {
-    /* disable usb interrupt */
-    PRR1 |= 1<<PRUSB;
-
-    atmega_set_prescaler(CPU_ATMEGA_CLK_SCALE_INIT);
     avr8_stdio_init();
     cpu_init();
     led_init();

boards/common/atmega/board.c

@@ -22,21 +22,10 @@
 #include "cpu.h"
 #include "periph/gpio.h"
 
-#ifndef CPU_ATMEGA_CLK_SCALE_INIT
-#define CPU_ATMEGA_CLK_SCALE_INIT    CPU_ATMEGA_CLK_SCALE_DIV1
-#endif
-
 void led_init(void);
 
 void __attribute__((weak)) board_init(void)
 {
-#ifdef CPU_ATMEGA32U4
-    /* disable usb interrupt on Atmega32U4 */
-    PRR1 |= 1<<PRUSB;
-#endif
-
-    atmega_set_prescaler(CPU_ATMEGA_CLK_SCALE_INIT);
-
     cpu_init();
 #ifdef LED0_ON
     led_init();

cpu/atmega_common/atmega_cpu.c

@@ -28,12 +28,17 @@
 
 #include <avr/pgmspace.h>
 
+#include "board.h"
 #include "cpu.h"
 #include "panic.h"
 
 #define ENABLE_DEBUG 0
 #include "debug.h"
 
+#ifndef CPU_ATMEGA_CLK_SCALE_INIT
+#define CPU_ATMEGA_CLK_SCALE_INIT    CPU_ATMEGA_CLK_SCALE_DIV1
+#endif
+
 extern uint8_t mcusr_mirror;
 extern uint8_t soft_rst;
 
@@ -62,6 +67,11 @@ void avr8_reset_cause(void)
 #endif
 }
 
+void __attribute__((weak)) avr8_clk_init(void)
+{
+    atmega_set_prescaler(CPU_ATMEGA_CLK_SCALE_INIT);
+}
+
 /* This is a vector which is aliased to __vector_default,
  * the vector executed when an ISR fires with no accompanying
  * ISR handler. This may be used along with the ISR() macro to

cpu/atxmega/include/cpu_conf.h

@@ -63,11 +63,6 @@ extern "C" {
  */
 #define IRQ_API_INLINED                 (1)
 
-/**
- * @brief   This arch require special clock initialization.
- */
-#define CPU_AVR8_HAS_CLOCK_INIT          1
-
 #ifdef __cplusplus
 }
 #endif

cpu/avr8_common/avr8_cpu.c

@@ -31,9 +31,7 @@
 #include <avr/pgmspace.h>
 
 #include "cpu.h"
-#ifdef CPU_AVR8_HAS_CLOCK_INIT
 #include "cpu_clock.h"
-#endif
 #include "board.h"
 #include "irq.h"
 #include "periph/init.h"
@@ -91,14 +89,17 @@ void get_mcusr(void)
 
 void cpu_init(void)
 {
+#ifdef PRUSB
+    /* disable usb interrupt */
+    PRR1 |= 1<<PRUSB;
+#endif
+
     avr8_reset_cause();
 
     wdt_reset();   /* should not be nececessary as done in bootloader */
     wdt_disable(); /* but when used without bootloader this is needed */
 
-#ifdef CPU_AVR8_HAS_CLOCK_INIT
     avr8_clk_init();
-#endif
 
     /* Initialize stdio before periph_init() to allow use of DEBUG() there */
 #ifdef MODULE_AVR_LIBC_EXTRA