rootfs.py: catch inner warn message

rootfs.py: catch inner warn message

Package managements (smart/apt-get/opkg-cl) generate some warn messages
to stdout, and we need to catch them and output by bb.warn.

Here is an example, while invoking smart to attempt install doc packages,
if install failed, it generates warn message to stdout.
...
|warning: Can't install util-linux-doc-2.24.2-r1@i586: Can't
install util-linux-doc-2.24.2-r1@i586: no package provides info
...

The fix catches it and outputs:
...
|WARNING: log_check: There is a warn message in the logfile
|WARNING: log_check: Matched keyword: [warn]
|WARNING: log_check: warning: Can't install util-linux-doc-2.24.2-r1@
i586: Can't install util-linux-doc-2.24.2-r1@i586: no package provides
info
...

(From OE-Core rev: f8d725f49f2be4b854f523a5ee3a5c4357e67e30)

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>