base.bbclass: Use subprocess rather than os.system for do_unpack

base.bbclass: Use subprocess rather than os.system for do_unpack

gzip reports broken pipe errors with do_unpack on Fedora with
certain builds of gzip and bash. By avoding python's SIGPIPE handler
we can work correctly on these distributions.

Patch based on a patch from the OE-devel mailing list, thanks to
Khem Raj <raj.khem@gmail.com> and Holger Freyther <zecke@selfish.org>

Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>