openharmony-image-common.inc: Have more image space to workaround ZFS based hosts

On ZFS hosts with compression on, the filesystem generation will fail.
The current default for extra image space is 1.2. Bump that to 2 to
workaround this issue and also have additional free space on the image.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>