boards: add atmega based boards clock scale defaults

The `atmega_set_prescaler` was using a "sensible" default
but it's better to define it at the board level to make
it clear.

boards/common/arduino-atmega/include/board_common.h

@@ -66,6 +66,13 @@ extern "C" {
 #define LED_PANIC           LED0_ON
 /** @} */
 
+/**
+ * @name CPU clock scale for arduino boards
+ *
+ */
+#define CPU_ATMEGA_CLK_SCALE_INIT    CPU_ATMEGA_CLK_SCALE_DIV1
+/** @} */
+
 /**
  * @name    xtimer configuration values
  * @{

boards/jiminy-mega256rfr2/include/board.h

@@ -90,6 +90,29 @@ extern "C" {
 #define XTIMER_HZ           (125000UL)
 /** @} */
 
+/**
+ * @name Indicate Watchdog cleared in bootloader an
+ *
+ * AVR CPUs need to reset the Watchdog as fast as possible.
+ * This flag indicates that the watchdog is reseted in the bootloader
+ * and that the MCUSR value is stored in register 2 (r2)
+ * @{
+ */
+#define BOOTLOADER_CLEARS_WATCHDOG_AND_PASSES_MCUSR 1
+/** @} */
+
+/**
+ * @name CPU clock scale for jiminy-megarfr256rfr2
+ *
+ * The CPU can not be used with the external xtal oscillator if the core
+ * should be put in sleep while the transceiver is in rx mode.
+ *
+ * It seems the as teh peripheral clock divider is set to 1 and this all
+ * clocks of the timer, etc run with 16MHz increasing power consumption.
+ */
+#define CPU_ATMEGA_CLK_SCALE_INIT    CPU_ATMEGA_CLK_SCALE_DIV1
+/** @} */
+
 /**
  * @brief Initialize board specific hardware, including clock, LEDs and std-IO
  */

boards/mega-xplained/include/board.h

@@ -118,6 +118,13 @@ extern "C" {
 #define FILTER_OUTPUT          GPIO_PIN(PORT_A, 7)
 /** @} */
 
+/**
+ * @name CPU clock scale for mega-xplained
+ *
+ */
+#define CPU_ATMEGA_CLK_SCALE_INIT    CPU_ATMEGA_CLK_SCALE_DIV1
+/** @} */
+
 /**
  * @brief   Initialize board specific hardware, including clock, LEDs and std-IO
  */

boards/waspmote-pro/include/board.h

@@ -150,6 +150,13 @@ extern "C" {
                                      MUX_USB_XBEE_ON
 /** @} */
 
+/**
+ * @name CPU clock scale for waspmote-pro
+ *
+ */
+#define CPU_ATMEGA_CLK_SCALE_INIT    CPU_ATMEGA_CLK_SCALE_DIV1
+/** @} */
+
 /**
  * @name    xtimer configuration values
  * @{