spl: Add a define for SPL_TPL_PROMPT

We should use a macro rather than hard-coding the SPL prompt to 'spl'
since the code can be used by TPL too. Add a macro that works for both
and use it in various places.

This allows TPL to use the same code without printing confusing messages.

Note that the string is lower case ('spl', 'tpl') which is a change from
previously.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Simon Glass <sjg@chromium.org>