Fix AppVeyor CI

- AppVeyor now uses vs2017 image
- Use nuget restore instead of dotnet restore
- Use msbuild -t:build -restore to get proper nuget package restoration in CompileSharpmake.bat

.appveyor.yml

@@ -1,6 +1,6 @@
 # Sharpmake AppVeyor CI configuration
 
-image: Visual Studio 2015
+image: Visual Studio 2017
 
 platform:
 - Any CPU
@@ -11,7 +11,7 @@ configuration:
 
 before_build:
 - cmd: bootstrap-sharpmake.bat
-- cmd: dotnet restore
+- cmd: nuget restore Sharpmake.sln
 
 build:
   project: Sharpmake.sln

CompileSharpmake.bat

@@ -29,32 +29,14 @@ echo MSBuild batch path: !VSMSBUILDCMD!
 call !VSMSBUILDCMD!
 if %errorlevel% NEQ 0 goto end
 
-call :NugetRestore %1 win
-if %errorlevel% NEQ 0 goto end
-
 call :BuildSharpmake %1 %2 %3
-if %errorlevel% NEQ 0 goto end
-
-
-:: Restore nuget
-:NugetRestore
-echo Restoring nuget packages for %~1
-
-set DOTNET_RESTORE=dotnet restore "%~1"
-if "%~2" neq "" set DOTNET_RESTORE=%DOTNET_RESTORE% -r %2
-echo %DOTNET_RESTORE%
-%DOTNET_RESTORE%
-if %errorlevel% NEQ 0 (
-    echo ERROR: Failed to restore nuget package for %~1
-    exit /b 1
-)
-exit /b 0
+goto end
 
 :: Build Sharpmake using specified arguments
 :BuildSharpmake
 echo Compiling %~1 in "%~2|%~3"...
 
-set MSBUILD_CMD=msbuild "%~1" /nologo /verbosity:quiet /p:Configuration="%~2" /p:Platform="%~3"
+set MSBUILD_CMD=msbuild -t:build -restore "%~1" /nologo /verbosity:quiet /p:Configuration="%~2" /p:Platform="%~3"
 echo %MSBUILD_CMD%
 %MSBUILD_CMD%
 if %errorlevel% NEQ 0 (

bootstrap-sharpmake.bat

@@ -13,24 +13,8 @@ echo %SM_CMD%
 %SM_CMD%
 if %errorlevel% NEQ 0 goto error
 
-call :NugetRestore Sharpmake.sln win
-if %errorlevel% NEQ 0 goto error
-
 goto success
 
-:NugetRestore
-echo Restoring nuget packages for %~1
-
-set DOTNET_RESTORE=dotnet restore "%~1"
-if "%~2" neq "" set DOTNET_RESTORE=%DOTNET_RESTORE% -r %2
-echo %DOTNET_RESTORE%
-%DOTNET_RESTORE%
-if %errorlevel% NEQ 0 (
-    echo ERROR: Failed to restore nuget package for %~1
-    exit /b 1
-)
-exit /b 0
-
 @REM -----------------------------------------------------------------------
 :success
 COLOR 2F