Add Sanguino 644p to ENV for PlatformIO

platformio.ini

 board = sanguino_atmega1284p
 upload_speed = 57600
 lib_deps = ${common.lib_deps}
+
+[env:sanguino_atmega644p]
+platform = atmelavr
+framework = arduino
+board = sanguino_atmega644p
+lib_deps = ${common.lib_deps}