imgbase: Add layer --current

You can now find out which layer you are currently booted into

Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>
plugins: Add kexec stub

The idea of the kexec plugins is to be able to reboot into an image
quickly.

Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>
build: Fixx mirror downloads

Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>