imgbase: Make base read-only after creation

Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>
imgbase: Move bootloader stuff into class

Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>
Add some more infos to the README

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