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-lowpower/README

@@ -0,0 +1,18 @@
+This is _functionally_ identical to the "button-irq-printf"
+example, but has been modified to use some low power features.
+
+There is a 115200@8n1 console on PA2, which prints a tick count every second,
+and when the user push button is pressed, the time it is held down for is
+timed (in milliseconds)
+
+Instead of free running timers and busy loops, this version uses the RTC
+module and attempts to sleep as much as possible, including while the button
+is pressed.
+
+Status
+~~~~~~
+Only very basic power savings are done!
+
+Current consumption, led off/on, 16Mhz HSI:    2.7mA/5.4mA
+Current consumption, led off/on,  4Mhz HSI:    1.4mA/?.?mA
+Current consumption, led off/on,  4Mhz MSI:    0.9mA/?.?mA