presence: Add fallback redundancy policy

Add a fallback redundancy implementation of the RedundancyPolicy
interface.

The fallback policy associates multiple PresenceSensor instances
to a single fan, and "falls-back" on secondary sensor implementations
when the primary sensor cannot see the fan.

Change-Id: I6468d77d97b8916b3ff33bcd0cd28a102d1aaba1
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/presence/Makefile.am b/presence/Makefile.am
index 2a5976c..33f13a3 100644
--- a/presence/Makefile.am
+++ b/presence/Makefile.am
@@ -5,6 +5,7 @@
 	phosphor-fan-presence-tach
 
 phosphor_fan_presence_tach_SOURCES = \
+	fallback.cpp \
 	fan.cpp \
 	gpio.cpp \
 	tach.cpp \