scripts: 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]

(From OE-Core rev: 57f843146ed62c04c23bc380dc8cb38aba264f1c)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/swabber-strace-attach

@@ -1,6 +1,7 @@
 #!/usr/bin/env python
 import os
 import sys
+import subprocess
 
 # Detach from the controlling terminal and parent process by forking twice to daemonize ourselves,
 # then run the command passed as argv[1]. Send log data to argv[2].
@@ -24,7 +25,7 @@ os.dup2(si.fileno(), sys.stdin.fileno())
 os.dup2(so.fileno(), sys.stdout.fileno())
 os.dup2(se.fileno(), sys.stderr.fileno())
 
-ret = os.system(sys.argv[1])
+ret = subprocess.call(sys.argv[1], shell=True)
 
 os._exit(ret)