avr: added initial module to handle reed contacts.

Detects and reports opening and closing of a window/door.
Nexts step is to allow for external configuration (e.g. standard mode
with one reed contact versus multi-contact mode to detect tilting of a
window/door).

avr/modules/0BUILD

@@ -10,6 +10,7 @@
     flash
     lcd
     led
+    reed
     si7021
     stats
     timer