tools: mxsimage: Fix build with OpenSSL 1.1.x

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().

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@konsulko.com>