[build-in-chroot.sh] Add script for building packages in a chroot.

* Used for my Jenkins CI server
* TODO: Make sure package signing works
* TODO: Enable the creation of chroots for architectures different from the host
[.gitignore] Ignore the configuration file for build-in-chroot.sh
[build-in-chroot.sh] Disable colored output in makepkg and show commands being run
[build-in-chroot.sh] Fix bug that causes install file to not be copied when the packaging sources are not in the current directory