* apcssh: add descriptive error when required file(s) not passed
* create: dont create key+cert file when key isn't supported by NMC2
* config: fix usage messages re: key types
* p15 files: dont generate key+cert when it isn't needed (aka NMC2 doesn't support key)
* pkcs15: pre-calculate envelope when making the p15 struct
* pkcs15: omit key ID 8 & 9 from EC keys
* pkcs15: update key decode logic
* pkcs15: add key type value for easy determination of compatibility
* pkcs15: add ec key support
* pkcs15: separate functions for key and key+cert p15 files
* update README
see: https://github.com/gregtwallace/apc-p15-tool/issues/6
The NMC Security Wizard can also produce .p15 files that contain just a private key. Add this ability to this tool.
When the `create` function is used, both files will be outputted.
