This function will be called from x509_check_for_self_signed() and
pkcs7_verify_one(), which will be imported from linux in a later patch.
While it does exist in linux code and has a similar functionality of
rsa_verify(), it calls further linux-specific interfaces inside.
That could lead to more files being imported from linux.
So simply re-implement it here instead of re-using the code.
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
b2a1049b5c