The EVP_MD_CTX and EVP_CIPHER_CTX are made opaque since 1.1.x , so instead
of embedding them directly into struct sb_image_ctx and initializing them
using EVP_*_CTX_init(), we use pointers and allocate the crypto contexts
using EVP_*_CTX_new().

This is an adoption of the U-Boot commit 7bae13b7579a6b from Marek Vasut

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>