[efi] Write out PE header only after writing sections

[efi] Write out PE header only after writing sections

Hybrid bzImage and UEFI binaries (such as wimboot) include a bzImage
header within a section starting at offset zero, with the PE header
effectively occupying unused space within this section.

Allow for this by treating a section placed at offset zero as hidden,
and by deferring the writing of the PE header until after the output
sections have been written.

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