meta: replace os.popen with subprocess.Popen

meta: replace os.popen with subprocess.Popen

Replace os.popen with subprocess.Popen since the older function would
fail (more or less) silently if the executed program cannot be found

There are both bb.process.run() and bb.process.Popen() which wraps the
subprocess module, use it for simplifying the code.

Note: We don't need the "2>/dev/null" or "2>&1" since bb.process.run()
can handle it, it will raise exception when error occurs, we should
handle the exception ourselves if we want to ignore the error.

More info:
http://docs.python.org/library/subprocess.html#subprocess-replacements

[YOCTO #2454]

(From OE-Core rev: e83d8e58a6b107eea87df0ec233a1bc932b2c6ea)

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