spl: support TPL_STACK, TPL_MAX_SIZE and TPL_TEXT_BASE via Kconfig

Let's clean up behind ourselves and move the (newly defined)
TPL_STACK, TPL_MAX_SIZE and TPL_TEXT_BASE into Kconfig.  Given that
0x0 might be considered to be valid values for TPL_TEXT_BASE and
TPL_STACK, we need to introduce helper config options
("TPL_NEEDS_SEPARATE_...") to indicate that these symbols are used
(and not inherited from their SPL variants) for any given
target-platform.

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