UBUNTU: [Packaging] add rules to build FIT image

BugLink: https://bugs.launchpad.net/bugs/1847969



Add a fit_signed option and a script so we can build FIT images that
will be eventually signed.

Signed-off-by: Alfonso Sánchez-Beato <alfonso.sanchez-beato@canonical.com>
Signed-off-by: Shrirang Bagul <shrirang.bagul@canonical.com>
Acked-by: Kleber Souza <kleber.souza@canonical.com>
Acked-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
(cherry picked from commit 517c893100a8c9315c6469a5e8ec74b8b385d321)
Signed-off-by: Shrirang Bagul <shrirang.bagul@canonical.com>