meta/lib/oe/utils.py: Corrected the return value of both_contain()
oe.utils.both_contain() should return the result as "checkvalue" or "",
but the latest implement returns as "set(['checkvalue'])" or "";
It causes that bitbake.conf generates the wrong result of COMBINED_FEATURES,
which contains the common components in both DISTRO_FEATURE and MACHINE_FEATURES.
For example, build in Dizzy branch, COMBINED_FEATURES is "alsa usbhost ...",
but recently, COMBINED_FEATURES is like "set(['alsa']) set(['usbhost']) ...".
Signed-off-by: Jun Zhu <R01007@freescale.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
