Add phosphor-image-signing if verify_signature is enabled

The phosphor-image-signing adds the public key to the BMC rootfs
and it was added if the UBI distro was enabled.
The phosphor-software-manager supports signature verification
regardless of the choice of BMC layout, so add it based if the
signature verification config (verify_signture) is enabled.

Otherwise the user would need to know that the package
phosphor-image-signing needed to be added if the signature
verification config was enabled, which is not intuitive. This way
it's done automatically.

Tested: Verified that adding verify_signature resulted in the
        /etc/activationdata/ directory being added to the BMC
        rootfs, and without it no directory was created.

(From meta-phosphor rev: 379f327e91542e7b2c3c4fb6b41b325b1a0a3412)

Change-Id: If0137b75f6de3b3087389c98229c85c346804912
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>