env: remove callback.o for an SPL build

env: remove callback.o for an SPL build

env.h says this about about callback declarations (U_BOOT_ENV_CALLBACK):

 * For SPL these are silently dropped to reduce code size, since environment
 * callbacks are not supported with SPL.

So env_callback_init() does a lot of work to not find anything in the
guaranteed empty env_clbk list. Drop callback.o entirely from the link
and stub out the only public function defined in callback.o. This cuts
about 600 bytes from the SPL on my ppc build.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>