lib/rsa: Use EVP_PKEY instead of RSA

Most modern OpenSSL engines have methods overridden at the EVP level rather
than at RSA level, to make these engines work properly with mkimage, the RSA
signing code needs to switch to using EVP_* APIs as much as possible.

Signed-off-by: Donald Chan <hoiho@lab126.com>
[trini: Rebase on top of keyfile changes]
Signed-off-by: Tom Rini <trini@konsulko.com>