Add additional Sanguino bard versions to boards.txt

ATmega644P 16MHz, ATmega1284p 8, 16, 20MHz
Switched to Sanguino 0023r4

This is the actual Sanguino release for Arduino 023.
It has full ATmega1284p support.

Reference:
https://code.google.com/p/sanguino/downloads/detail?name=Sanguino-0023r4.zip&can=2&q=
Add bootloader files for ATmega1284/Sanguino

Added the optiboot bootloader, uses only 512kB of FLASH
Includes .hex file is for 20MHz µC Clock and serial speed of 57k6
Merge pull request #408 from STB3/Marlin_with_ATmega1284p_Sanguino_on_023_support

Marlin with ATmega1284p sanguino support under Arduino 023
Fix for incorrect step rate where acceleration rate is significantly greater than nominal speed - as seen with fast extrude/retract moves.
Merge pull request #1 from illuminarti/Fix_E_Speed

Fix step rate bug (especially for retraction)
Merge pull request #1 from illuminarti/Fix_E_Speed

Fix step rate bug (especially for retraction)
Merge branch 'Marlin_v1' of github.com:ErikZalm/Marlin into Marlin_v1