[efi] Extend PE header size to cover space up to first section

[efi] Extend PE header size to cover space up to first section

Hybrid bzImage and UEFI binaries (such as wimboot) may place sections
at explicit offsets within the PE file, as described in commit b30a098
("[efi] Use load memory address as file offset for hybrid binaries").
This can leave a gap after the PE headers that is not covered by any
section.  It is not entirely clear whether or not such gaps are
permitted in binaries submitted for Secure Boot signing.

To minimise potential problems, extend the PE header size to cover any
space before the first explicitly placed section.

Signed-off-by: Michael Brown <mcb30@ipxe.org>