efi: Provide a shim for load_image, start_image, unload_image

efi: Provide a shim for load_image, start_image, unload_image

Provide custom implementations of load_image(), start_image(),
and unload_image() to workaround shim just forwarding those
calls to the firmware.

The code consumes a PE-COFF image loaded into memory. The functions

* check validity of header
* copy the sections
* relocate the code
* invalidate the instruction cache
* execute the image
* return to caller

This was previously in use in Ubuntu on riscv64 and arm64 only,
exposed as a single function  grub_efi_run_image(). It was
originally written by Heinrich and split up into 3 functions
by Julian to integrate with the upstream boot loader.

Caveats:

- We do not always check for over and underflows, but at the
  point we reach this loader, the file has been verified by
  shim already, so this is not much of a concern.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Julian Andres Klode <julian.klode@canonical.com>
Gbp-Pq: Topic secure-boot
Gbp-Pq: Name efi-use-peimage-shim.patch