meta-openpower: Predicate packages on MACHINE_FEATURES

Change packages enabled by meta-openpower to be enabled if and only if
both its corresponding feature string (obmc-openpower,
obmc-openpower-extra, obmc-openpower-legacy) are present in both
MACHINE_FEATURES and DISTRO_FEATURES.

Fixes openbmc/openbmc#2652.

Change-Id: Ife1a9b08e94bd85a888d584a0caa4dbfa6a172c0
Signed-off-by: Xo Wang <xow@google.com>