Replace os.system with subprocess.call since the older function would
fail (more or less) silently if the executed program cannot be found
More info:
http://docs.python.org/library/subprocess.html#subprocess-replacements
[YOCTO #2454]
(From OE-Core rev: a07d03cc6f67c88feb9813ae7deb6e4a93552dfe)
Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>