extract_v2: use crypto abstraction, and clean up code

- put generic crypto function to own file
- put openssl specific crypto implementations into its own file
- remove EVP_MD_CTX_FLAG_ONESHOT setting as it was mere optimization
  (only useful for Padlock SHA accelerator IIRC)
- use separate identity generation hash context
- remove usage of gotos
- remove unused and unneeded modes
- harden extract_v2 file parser while at it