bitbake.conf: use .ubifs.img extension in IMAGE_CMD_ubi

bitbake.conf: use .ubifs.img extension in IMAGE_CMD_ubi

The current IMAGE_CMD_ubi creates an interim image with a .rootfs.ubifs
extension.

The ubifs image created is not considered a valid rootfs without a UBI
container.

Change the filename of the iterim ubifs image used by IMAGE_CMD_ubi to
.ubifs.img to match the IMAGE_CMD_ubifs command.

Signed-off-by: Ben Gardiner <bengardiner@nanometrics.ca>
Acked-by: Denys Dmytriyenko <denys@ti.com>
Signed-off-by: Tom Rini <tom_rini@mentor.com>