extension-release.d/: add a new field SYSEXT_SCOPE= for clarifying what a system...

extension-release.d/: add a new field SYSEXT_SCOPE= for clarifying what a system extension is for

This should make things a bit more robust since it ensures system
extension can only applied to the right environments. Right now three
different "scopes" are defined:

1. "system" (for regular OS systems, after the initrd transition)
2. "initrd" (for sysext images that apply to the initrd environment)
3. "portable" (for sysext images that apply to portable images)

If not specified we imply a default of "system portable", i.e. any image
where the field is not specified is implicitly OK for application to OS
images and for portable services – but not for initrds.