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>