config: add build config option to sign each .apk package

config: add build config option to sign each .apk package

Add a build config option to sign each individual .apk package.

If individual .apk files are signed with the build key, they can be
installed with 'apk add' without '--allow-untrusted' to a firmware
compiled by the same buildhost.

Enable the option by default, but disable it for BUILDBOT.

(At the moment, since commit 084697e, only the package index is signed,
which forces users to use '--allow-untrusted' when installing
self-built .apk files.)

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>