UBUNTU: [Packaging] build-fit: specify the kernel version

Need to tell mkinitramfs which kernel we would like to work on otherwise
it will use the one that is running.

Signed-off-by: Wen-chien Jesse Sung <jesse.sung@canonical.com>
(cherry picked from commit cf35df06421b1c68417d59bb7579f0919bc60dae)
Signed-off-by: Shrirang Bagul <shrirang.bagul@canonical.com>