Add examples making use of the RTC

Add an example using the RTC to help with a lower power design.  This is
a sister example to the existing "button-irq-printf", which is
functionally identical, but uses far less power.

There's more tricks that can be done to lower the power even further,
but this shows a few of the early steps that can be done, using the RTC
wakeup instead of a timer.

examples/stm32/l1/stm32l-discovery/button-irq-printf/syscfg.h

@@ -24,8 +24,8 @@
 extern "C" {
 #endif
 
-#include <libopencm3/stm32/l1/gpio.h>
-#include <libopencm3/stm32/l1/nvic.h>
+#include <libopencm3/cm3/nvic.h>
+#include <libopencm3/stm32/gpio.h>
 #include <libopencm3/stm32/exti.h>
 #include <libopencm3/stm32/usart.h>
 
@@ -46,6 +46,7 @@ extern "C" {
 
 	struct state_t {
 		bool falling;
+		int tickcount;
 	};