efi: libstub: Merge zboot decompressor with the ordinary stub

efi: libstub: Merge zboot decompressor with the ordinary stub

Even though our EFI zboot decompressor is pedantically spec compliant
and idiomatic for EFI image loaders, calling LoadImage() and
StartImage() for the nested image is a bit of a burden. Not only does it
create workflow issues for the distros (as both the inner and outer
PE/COFF images need to be signed for secure boot), it also copies the
image around in memory numerous times:
- first, the image is decompressed into a buffer;
- the buffer is consumed by LoadImage(), which copies the sections into
  a newly allocated memory region to hold the executable image;
- once the EFI stub is invoked by StartImage(), it will also move the
  image in memory in case of KASLR, mirrored memory or if the image must
  execute from a certain a priori defined address.

There are only two EFI spec compliant ways to load code into memory and
execute it:
- use LoadImage() and StartImage(),
- call ExitBootServices() and take ownership of the entire system, after
  which anything goes.

Given that the EFI zboot decompressor always invokes the EFI stub, and
given that both are built from the same set of objects, let's merge the
two, so that we can avoid LoadImage()/StartImage but still load our
image into memory without breaking the above rules.

This also means we can decompress the image directly into its final
location, which could be randomized or meet other platform specific
constraints that LoadImage() does not know how to adhere to. It also
means that, even if the encapsulated image still has the EFI stub
incorporated as well, it does not need to be signed for secure boot when
wrapping it in the EFI zboot decompressor.

In the future, we might decide to retire the EFI stub attached to the
decompressed image, but for the time being, they can happily coexist.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>