rsa-verify: Rework host check for CONFIG_RSA_VERIFY_WITH_PKEY

rsa-verify: Rework host check for CONFIG_RSA_VERIFY_WITH_PKEY

While we do not want to use CONFIG_RSA_VERIFY_WITH_PKEY on the host, we
cannot undef the symbol in this manner. As this ends up being a test
within another function we can use !tools_build() as a test here.

Cc: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>