Fix for native-image to work in Windows

[sshark]

• The final native-image executable cannot be located in the file
  native-image.cmd unless the full path to native-image.cmd is
  given. Therefore, a new parameter is provided to the user to
  specify the native-image.cmd location.
• Picks the right CLASSPATH separator according to the OS
  instead of hardcoded to colon. Using Colon will fail in Windows
  build.
• Tested against GraalVM 20.0.0 and VS 2019.

[lightbend-cla-validator]

Hi @sshark,

Thank you for your contribution! We really value the time you've taken to put this together.

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[muuki88]

Thanks for the very well done pull request ❤️

A small remark that would be nice if you could fix it 😄

[muuki88]

Thanks for the fix ❤️ I'll merge this as soon as the checks are green.

This might be too much to ask, but since windows users are rare 😄
If you have time and interest in helping to fix the windows build that would be awesome.
However don't feel pressured or anything. I just wanted to ask 😃

The appveyor build started to fail for no reasons a month ago. It started with #1299 which didn't change any windows related stuff.
Example: https://ci.appveyor.com/project/muuki88/sbt-native-packager/builds/31212944

[info] [error] > call set _PARAM1=%1 
[info] [error] error < > was unexpected at this time.
[info] [error] 
[info] [error] >   set "_TEST_PARAM=\"-Dtest.hoge=\[]!< >%\""
[info] [error] Total time: 13 s, completed Mar 3, 2020 4:15:24 PM
[error] x windows / test-bat-template 
[error]  Cause of test exception: {line 4}  Command failed: checkScript failed
Running windows / test-custom-main
[error] java.lang.RuntimeException: Failed tests:
[error] 	windows / test-bat-template
[error] 

I can't make any sense from the error message. So if you can and have an idea how to fix this, I would be suuuuper happy 😍

[sshark]

Let me take a shot at it over the weekend and see if I can make sense out of those error messages. I am not a Windows developer either. For me, Windows is just a means to an end 🤐 😃

[muuki88]

Let me take a shot at it over the weekend and see if I can make sense out of those error messages. I am not a Windows developer either. For me, Windows is just a means to an end

Even more thanks then 😍