4.2.0

• Updated MCUdude's cores (#302):
  • MajorCore v2.2.2 (release notes)
  • MegaCore v2.2.3 (release notes)
  • MightyCore v2.2.2 (release notes)
  • MiniCore v2.2.2 (release notes)
  • MicroCore v2.3.0 (release notes)
• Added initial support for the Urboot bootloader for MicroCore (#311)
• Fixed an issue with missing tool-avrdude package when bootloader target is selected (#269)

Thanks @MCUdude for your great contributions!