32u4 board fixes; Feather M0 fixes; nRF52840 added
- Fixed Feather M0 Proto UF2 entries; didn't work properly with Arduino.
- Fixed 32u4 board entries: Circuit Playground Classic, Feather 32u4, Flora, and Bluefruit Micro. Since 2.3.0 they didn't work with Arduino or code.org Maker App.
- Added nRF52840 entries.