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/Makefile

@@ -0,0 +1,24 @@
+##
+## This file is part of the libopencm3 project.
+##
+## Copyright (C) 2009 Uwe Hermann <uwe@hermann-uwe.de>
+##
+## This library is free software: you can redistribute it and/or modify
+## it under the terms of the GNU Lesser General Public License as published by
+## the Free Software Foundation, either version 3 of the License, or
+## (at your option) any later version.
+##
+## This library is distributed in the hope that it will be useful,
+## but WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+## GNU Lesser General Public License for more details.
+##
+## You should have received a copy of the GNU Lesser General Public License
+## along with this library.  If not, see <http://www.gnu.org/licenses/>.
+##
+
+BINARY = main
+LDSCRIPT = ../../../../../lib/stm32/l1/stm32l15xxb.ld
+
+include ../../Makefile.include
+