spl: consistently use $(SPL_TPL_) to select features for SPL and TPL builds

To allow for a finer-grained control of features for TPL and SPL
builds all modules/boot-methods/etc. need to be consistently selected
based on the $(SPL_TPL_) macros.

This allows splitting the associated config-options in Kconfig: we
don't split the Kconfig options here and now, as this should happen on
an as-needed basis, whenever someone needs a feature/boot-method/etc.
in their TPL.

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