efi/libstub: zboot: Avoid eager evaluation of objcopy flags

The Make variable containing the objcopy flags may be constructed from
the output of build tools operating on build artifacts, and these may
not exist when doing a make clean.

So avoid evaluating them eagerly, to prevent spurious build warnings.

Suggested-by: Pedro Falcato <pedro.falcato@gmail.com>
Tested-by: Alan Bartlett <ajb@elrepo.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>