Add chroot building support to hpkgbuild

[scottmc]

In order to avoid unintended picking-up of dependencies, the build process
should run in a dedicated chroot-environment, so hpkgbuild would have to
create a basic folder structure, bind-mount /system into it, invoke pkgman
in order to download the build-required packages, put the downloaded
packages into the chroot-environment and mount them there via package-fs.
Finally, hpkgbuild would proceed with the usual: i.e. download the
sources, enter the chroot and execute whatever build action has been
requested.