When building diskless images for deployment the kernel owning the chroot
may be different from the kernel the chroot is to deployed on.
In such cases the default of falling back to $(uname -r) results in a image that
cannot be deployed.

For such deployment a common.postinst is still used to run dkms build/install with
the correct kernel. So prefer to run common.postinst if it exists.

common.postinst script is used when building in chroot for
diskless deployment.

Fixes: a2b5118f2a ("LU-18533 build: fix the common.postinst nonexistence issue")