aiaiai-test-patchset: optimize disk space

aiaiai-test-patchset: optimize disk space

When compiling multiple defconfigs, remove the build tree between
each compilation to spare disk space.

In this way the build trees don't accumulate on the disk while
testing the same patchset, and the patchset scales with the
number of defconfigs, which is not limiting the build anymore.

Jacob:
  Honor preserve variable, so that when requested we maintain files.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>