Use OpenSSL libcrypto for MD5 checking

* Use OpenSSL libcrypto static library for both backup and restore md5
  operations
* Warn users when nandroid.md5 is missing entries on restore
* Only verify md5 checksums for partitions selected for restore instead
  of verifying checksums for all files even if not selected for restore
* Remove unneeded nandroid-md5.sh script