MightyCore v3.0.0

Changelog:

• Replace Optiboot with Urboot
• Replace Optiboot flash library with one that supports Urboot
• Enable LTO by default
• Add -fpermissive GCC build flag but better compatibility
• Add support for EEPROM uploads directly from the sketch using Urboot or an ISP programmer
  • See README for more information
• Add 6 MHz external oscillator option
• Update toolchain to use Avrdude 7.2