kernel.bbclass: replace os.system with subprocess.call

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]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>