kernel.bbclass: replace os.system with subprocess.call
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 <58c339bb2d6e36747c1485f8911298e2be82d9f6@windriver.com>
Signed-off-by: Richard Purdie <3b7a1db6efedd40440bc6443bf377195011524e5@linuxfoundation.org>
Signed-off-by: Martin Jansa <516df3ff67e1fa5d153800b15fb204c0a1a389dc@gmail.com>
Signed-off-by: Koen Kooi <42e3d96910f922bdd7dd1805fe3f8aff634f391d@dominion.thruhere.net>

WindRiver-OpenSourceLabs/meta-oe-subset,WindRiver-OpenSourceLabs/meta-oe-subset