Kpm pack generate bash scripts

parent #327 
This batch file will always run on Windows, no matter which platform generates it. Although Windows can deal with both `/` and `\`, I am using hard-coded `\` to make it explicit that we are generating batch file for Windows.

The code generating bash script is symmetric, using hard-coded `/`:
https://github.com/aspnet/KRuntime/pull/788/files#diff-1d4ec359a00b16ff44a167b9d7df009dR124