base.bbclass: Replace os.system with subprocess call.

base.bbclass: Replace os.system with subprocess call.

Often gzip is reporting broken pipe errors with do_unpack of
tar.gz files.

If you use the commands described above to extract a tar.gz file, gzip
sometimes emits a Broken pipe error message. This can safely be ignored
if tar extracted all files without any other error message.

We do not let python install its SIGPIPE handler and use subprocess call
to invoke the command.

This is based on the following python bug report.
http://bugs.python.org/issue1652

Signed-off-by: Khem Raj <raj.khem@gmail.com>