clarify location of cloned repository for linux builds

In case the cloned repository is out of the home directory, starting
the schroot can fail with an error "Failed to change to directory".
use SUDO_USER if "logname" does not return any result

This can happen in certain situations e.g. when you enter an
OpenVZ container via "vzctl enter CTID" the login name is not
available.
add --force and --verbose to the XZ compression options

This will generate the output even if it already exists and show
additional information during the compression.
WIP: add support for OS X in the build script

X-Authored-By: @mn4367
X-Authored-By: @npinchot