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 <[email protected]>
Cc: Simon Glass <[email protected]>
Signed-off-by: Tom Rini <[email protected]>
Reviewed-by: Simon Glass <[email protected]>