lib: rsa: take spl/non-spl into account when building rsa_verify_with_pkey()

lib: rsa: take spl/non-spl into account when building rsa_verify_with_pkey()

Right now in multiple places there are only checks for the full
CONFIG_RSA_VERIFY_WITH_PKEY option, not split into main,spl,tpl variants.

This breaks when the rsa functions get enabled for SPL, for example to
verify u-boot proper from spl.

So fix this by using the existing helpers to distinguis between
build-steps.

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