add contrast adjust function and support for a new board

Some graphical LCD allow for software contrast adjustment. This patch add a menu item and a M-command for this function.

I have developed a printer controller board and a new user panel with a graphical LCD. Right now I am working on the sample approval for the first production samples and will start selling them quite soon. (the linked wiki pages will be setup soon)

I will ship the boards with a recent version of your Marlin firmware, so I added support for the board and the used LCD to the build tree.

The board is a slightly improved Melzi version with a ATmega1284p. The display is a graphical LCD with 128x64 pixel and a ST7565 controller, very, very similar to the dogm128 used by Dirk for his STB board. So the software is the same.

I have added a function for contrast control: A menu item in the control menu and a new M-command (M250 Cxxx). The contrast value is stored in the EEPROM, so changed the EEPROM data version to v08. This should work for  Dirks STB display as well.

I my repository this pull request is seperated into two individual commits, one for the contrast function, and one for the added hardware support, but the pull request seem to merge it all in one.

Thank you for all your good work so far!

  Michael