build: separate signing logic

build: separate signing logic

This separates the options for signature creation and verification

* SIGNED_PACKAGES create Packages.sig
* SIGNED_IMAGES add ucert signature to created images
* CHECK_SIGNATURE add verification capabilities to images
* INSTALL_LOCAL_KEY add local key-build to /etc/opkg/keys

Right now the buildbot.git contains some hacks to create images that
have signature verification capabilities while not storing private keys
on buildbot slaves. This commit allows to disable these steps for the
buildbots and only perform signing on the master.

Signed-off-by: Paul Spooren <mail@aparcar.org>