x-wic.inc: Default partition extra args to avoid expansion issues

When not defined, bitbake will not expand the variables and wic will be
passed unrecognized arguments. Here is an example:

unrecognized arguments: ${WIC_ROOTA_PARTITION_EXTRA_ARGS}

Fixes #69

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