makemake: Enable concurrent DEB builds

Two problems prevent us from building DEBs concurrently:

 * makedeb works in the %_builddir directory, and expects to be
   able to clear it out completely when it is finished.   For
   simplicity, change makedeb to build each package in its own
   temporary directory.
 * apt-ftparchive needs to update the Sources and Packages
   database files after each package is built.   It's not clear
   whether it's safe to run two instances of createrepo against the
   same database concurrently.  To prevent this, we wrap the call to
   createrepo with flock.

Signed-off-by: Euan Harris <euan.harris@citrix.com>