lib: rsa: distinguish between tpl and spl for CONFIG_RSA_VERIFY

lib: rsa: distinguish between tpl and spl for CONFIG_RSA_VERIFY

While the SPL may want to do signature checking this won't be
the case for TPL in all cases, as TPL is mostly used when the
amount of initial memory is not enough for a full SPL.

So on a system where SPL uses DM but TPL does not we currently
end up with a TPL compile error of:

    lib/rsa/rsa-verify.c:48:25: error: dereferencing pointer to incomplete type ‘struct checksum_algo’

To prevent that change the $(SPL_) to $(SPL_TPL_) to distinguish
between both. If someone really needs FIT signature checking in
TPL as well, a new TPL_RSA_VERIFY config symbol needs to be added.

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