Close inherited PIPE before doing work

Gmake in Darwin has file descriptor leak.
In a full build, ota_from_target_files will inherits
more than 2000 open PIPEs from gmake and fails in a call to select.select().
This change fixes the build by closing the PIPEs before doing real work.

Change-Id: Ie7035d7add0b1da3afb6bf9c2009d40f8c7d29b3