bootstage: adjust Makefile to allow including bootstage in SPL, but not in TPL

For timing our bootstages on the RK3368, which has a minimal TPL
(and where we consequently don't want to time the bootstages) and a
full-featured SPL (where we can bootstage recording), we need to
adjust the Makefile.

Use the $(SPL_TPL_) macro in the Makefile for bootstage.o

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>