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

mos.yml

@@ -44,8 +44,8 @@ config_schema:
   - ["app.config", "sonoff-basic.json"]
 
 
-build_vars:
-  FLASH_SIZE: 1048576
+#build_vars:
+#  FLASH_SIZE: 1048576
 
 # List of libraries used by this app, in order of initialisation
 libs:
@@ -56,6 +56,7 @@ libs:
   - origin: https://github.com/mongoose-os-libs/rpc-service-fs
   - origin: https://github.com/mongoose-os-libs/rpc-mqtt
   - origin: https://github.com/mongoose-os-libs/mqtt
+  - origin: https://github.com/mongoose-os-libs/dht
   - origin: libs/rpc-service-ota
 
 # Used by the mos tool to catch mos binaries incompatible with this file format