[efi] Remove redundant zero padding in PE header

[efi] Remove redundant zero padding in PE header

Hybrid bzImage and UEFI binaries (such as wimboot) require the PE
header to be kept as small as possible, since the bzImage header
starts at a fixed offset 0x1f1.

The PE header currently includes 128 bytes of zero padding between the
DOS and NT header portions.  This padding has been present since
commit 81d92c6 ("[efi] Add EFI image format and basic runtime
environment") first added support for EFI images in iPXE, and was
included on the basis of matching the observed behaviour of the
Microsoft toolchain.  There appears to be no requirement for this
padding to exist: EDK2 binaries built with gcc include only 64 bytes
of zero padding, Linux kernel binaries include 66 bytes of non-zero
padding, and wimboot binaries include no padding at all.

Remove the unnecessary padding between the DOS and NT header portions
to minimise the overall size of the PE header.

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