.oniro-ci: add new Linux hidden job definition

build-linux-image, build-linux-oniro-image-base and
build-linux-oniro-image-tests are new linux hidden jobs that defines
the flavor to be Linux and set the recipe to build to oniro-image-base
and oniro-image-base-tests respectively.

linux-qemu-x86 and linux-seco-imx8mm-c61-2gb extends them. This is
the first iteration and in later commits all the jobs will be
migrated to the new structure.

This is giving us a more precise control over the artifacts upload,
especially with the gcc and clang split.

This commit is not breaking any of the jobs that are not using the
new definition.

Signed-off-by: Davide Gardenal <davide.gardenal@huawei.com>