bitbake.conf: rename 'gobject-introspection-data' machine feature to 'qemu-usermode'

bitbake.conf: rename 'gobject-introspection-data' machine feature to 'qemu-usermode'

The new value is more general and better reflects what having the feature really means.
Introspection data, then, is built only if 'gobject-introspection-data' is in
DISTRO_FEATURES and 'qemu-usermode' is in MACHINE_FEATURES.

(From OE-Core rev: 9927a3d72e2272d8e3dc4785ba02e27802ee1c6c)

Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>