bmaptool: implement GPG signature verification

bmaptool: implement GPG signature verification

This is a feature which was requested by the Fedora communitiy.

Both clearsign and detached GPG signatures are supported. The signature file
auto-discovery is supported (similar to bmap file auto-discovery).

More user-facing information will be added to the man page a bit later.

We add the following command-line options:
    1. --bmap-sig option for specifying the detached signature file path
    2. --no-sig-verify option for making bmaptool avoid verifying the clearsign
       signature and avoid signature file auto-discovery

Change-Id: Id64194415759abdbbe4e2f54d997f0855f67c749
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>