When adding support for OpenSSL 1.1.x it was missed that EVP_PKEY_get0_RSA
is only available with that version. Earlier versions don't provide a fully
equivalent function, so add it to the compatibility helpers.

Fixes: 542a50d4eb (scripts: imx-image: fix build with OpenSSL 1.1.x)
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>