Added build cache in 'AppVeyor' for the Arduino SDK.

SDK won't be downloaded from the web if its' cache exists.

Author: MrPointer

appveyor.yml

@@ -5,13 +5,19 @@ environment:
   matrix:
   - ARDUINO_SDK_VERSION: 1.8.2
   - ARDUINO_SDK_VERSION: 1.8.6
+cache:
+- '%ARDUINO_SDK_PATH% -> %ARDUINO_SDK_VERSION%'
 install:
 - ps: cinst unzip
-- ps: $env:ARDUINO_SDK_FILE = "arduino-$env:ARDUINO_SDK_VERSION-windows.zip"
-- ps: $env:ARDUINO_SDK_URI = "https://downloads.arduino.cc/$env:ARDUINO_SDK_FILE"
-- ps: wget "$env:ARDUINO_SDK_URI" -O "$env:ARDUINO_SDK_FILE"
-- ps: unzip "$env:ARDUINO_SDK_FILE" -d "arduino-sdk"
 - ps: $env:ARDUINO_SDK_PATH = "$pwd\arduino-sdk\arduino-$env:ARDUINO_SDK_VERSION"
+- ps: $env:ARDUINO_SDK_FILE = "arduino-$env:ARDUINO_SDK_VERSION-windows.zip"
+- ps: |
+    if ( -Not (Test-Path -Path $env:ARDUINO_SDK_PATH) )
+    {
+      $env:ARDUINO_SDK_URI = "https://downloads.arduino.cc/$env:ARDUINO_SDK_FILE"
+      wget "$env:ARDUINO_SDK_URI" -O "$env:ARDUINO_SDK_FILE"
+      unzip "$env:ARDUINO_SDK_FILE" -d "arduino-sdk"
+    }
 - ps: $env:ARDUINO_SDK_PATH = ($env:ARDUINO_SDK_PATH -replace "\\","/")
 before_build:
 - ps: Copy-Item -Path $env:MINGW_PATH\mingw32-make.exe -Destination $env:MINGW_PATH\make.exe