arm: efi: add PE/COFF debug table to EFI header

This updates the PE/COFF header to emit the absolute path to the
decompressor vmlinux ELF file into a so-called NB10 Codeview entry.
This is hugely helpful when debugging the firmware->stub handover.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>