LightweightServo support for ATmega2560, renamings and bug fix

• LightweightServo support for ATmega2560.
• Renamed mCurrentMicrosecondsOrUnits to mLastTargetMicrosecondsOrUnits to make clear, that trim and reverse is NOT applied to this value.
• Changed DISABLE_MIN_AND_MAX_CONSTRAINTS to ENABLE_MIN_AND_MAX_CONSTRAINTS. Constraint checking is now disabled by default.
• candorgander fixed a bug in printEasingType() for non AVR platforms.