presence: Un-hardcode input device path

Allow a device path to be configured.

Change-Id: I2eabd71747b124eae96738d1332f58b96ece6dc9
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/presence/pfpgen.py b/presence/pfpgen.py
index 00ba900..187be32 100755
--- a/presence/pfpgen.py
+++ b/presence/pfpgen.py
@@ -139,6 +139,7 @@
     def __init__(self, *a, **kw):
         self.key = kw.pop('key')
         self.physpath = kw.pop('physpath')
+        self.devpath = kw.pop('devpath')
         kw['name'] = 'gpio-{}'.format(self.key)
         super(Gpio, self).__init__(**kw)