Andrei Gherzan authored Jun 09, 2021 and Stefan Schmidt committed Jun 11, 2021

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>