ukify: add new --extend= switch for importing an existing UKI's sections to later...

ukify: add new --extend= switch for importing an existing UKI's sections to later extend

This options is pretty simple, it allows specifying an UKI whose
sections to import first, and place at the beginning of the new UKI.

This is useful for generating multi-profile UKIs piecemeal: generate the
base UKI first, then append a profile, and another one and another one.

The sections imported this way are not included in any PCR signature,
the assumption is that that already happened before in the imported UKI.