Add DHT support.

Very rough stab just to test the waters -- this will need some
more work.

For now, hard code 4,5,15 pins (as per Sonoff SV), but before
rolling this out, add a config flag for this behavior.

Neat trick -- the presence of dht library in mos.yml triggers
inclusion of the code via define -DMGOS_HAVE_DHT=1

include/main.h

@@ -29,4 +29,6 @@ bool channel_get(int idx);
 int channel_get_total();
 void channel_handler(int gpio, void *arg);
 
+void dht_init();
+
 #endif // __MAIN_H