Update changelog with key updates
## v0.15.4-dev
+ * New features
+ * Changed signing keys to be base64 encoded so that they'd be easier to bake
+ into firmware and pass in environment variables on CI systems. The
+ previous raw binary format still works and will remain supported.
+ * Added commandline parameters for passing public and private keys via
+ commandline arguments. Along with the base64 change, this cleans up CI
+ build scripts.
+
* Bug fixes/Improvements
* Fix lseek seek_end issue on Mac when working with SDCards. This fixed an
issue where upgrade tasks didn't work on Macs. Not a common issue, but
