Only ignore unused functions when building for the ESP8266

This should be removed when the ESP8266 core is updated in PlatformIO: esp8266/Arduino#2881

Author: Lauszus

.travis.yml

@@ -63,7 +63,7 @@ env:
 
 install:
     - pip install -U platformio
-    - export PLATFORMIO_BUILD_FLAGS="$PLATFORMIO_BUILD_FLAGS -DDEBUG_USB_HOST -Wall -Werror -Wno-unused-function"
+    - export PLATFORMIO_BUILD_FLAGS="$PLATFORMIO_BUILD_FLAGS -DDEBUG_USB_HOST -Wall -Werror"
 
     #
     # Libraries from PlatformIO Library Registry:
@@ -74,4 +74,6 @@ install:
     - platformio lib install 62 416 417
 
 script:
-    - platformio ci --board=uno --board=due --board=teensy30 --board=teensy31 --board=teensy35 --board=teensy36 --board=teensylc --board=esp12e --board=nodemcu --lib="."
+    - platformio ci --lib="." --board=uno --board=due --board=teensy30 --board=teensy31 --board=teensy35 --board=teensy36 --board=teensylc
+    - export PLATFORMIO_BUILD_FLAGS="$PLATFORMIO_BUILD_FLAGS -Wno-unused-function" # Workaround https://github.com/esp8266/Arduino/pull/2881
+    - platformio ci --lib="." --board=esp12e --board=nodemcu