efi/libstub: add some missing EFI prototypes

Define the correct prototypes for the load_image, start_image and
unload_image boot service pointers so we can call them from the EFI
zboot code.

Also add some prototypes related to installation and deinstallation of
protocols in to the EFI protocol database, including some definitions
related to device paths.

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