package_manager: sign DEB package feeds

Implement debian package repository signature.
For each Release file created in repository subdirectory, a signature
Release.gpg is created.

Signature is performed using gpg backend when the following variables
are set in local.conf:
PACKAGE_CLASSES += "sign_package_feed"
PACKAGE_FEED_GPG_NAME = "<Id of GPG key>"
PACKAGE_FEED_GPG_PASSPHRASE_FILE="<path to password file>"

Signed-off-by: Xavier Berger <xavier.berger@bio-logic.net>
Signed-off-by: Ferry Toth <ftoth@exalondelft.nl>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>