UBUNTU: [Packaging] signing -- switch to linux-image as signed when available

UBUNTU: [Packaging] signing -- switch to linux-image as signed when available

When we are signing for an architecture switch to producing a
linux-image-unsigned package from the primary source.  linux-signed will
then produce the linux-image package which will be signed.  We expect
all consumers to use the signed form even if they are not utilising
the signature.

BugLink: http://bugs.launchpad.net/bugs/1764794
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>