rebuild buildfiles without build() only when needed

if buildfile has no build rules, it not make output package. So it
need only mark with build flag only when it has dependence change or
old output file exist.

Signed-off-by: Kalle Lampila <kalle.lampila@ixonos.com>
ensure outdated package output deletion

delete outdated package output, if there is not package output anymore.

Signed-off-by: Kalle Lampila <kalle.lampila@ixonos.com>
Merge pull request #36 from kallelampila/master

delete outdated package output, if there is not package output anymore.  rebuild buildfiles without build() only when needed.