Added Platformio support

- firmware can now be built in Platformio too
- minor bug fixes
- added LED board picture

.travis.yml

@@ -45,3 +45,14 @@ jobs:
       - export PATH=$HOME/arm-gcc-toolchain/bin:$PATH
       before_script: arm-none-eabi-gcc --version
 
+    # # Not yet supportted, see https://community.platformio.org/t/build-gd32-project-with-platformio/11944
+    # - name: platformio
+      # script: platformio run
+      # language: python
+      # python:
+      # - "2.7"
+      # install:
+      # - pip install -U platformio
+      # - platformio update
+      # cache:
+      # - directories: "~/.platformio"