stub: Also reserve sections for EFI stub

stub: Also reserve sections for EFI stub

The stub image may not have enough sections to cause enough PE header
space to be free for later expansion. Given that the stub is guaranteed
to be expanded we should always reserve enough sections for it.

This also bumps the reservation to 15. It doesn't add more space
compared to current builds but it more closely reflects the amount of
sections that a UKI could have in total if all optional sections are
used.

Fixes: #28593