Refactor MEGAFLASH/jtagflash

[lydon42]

The flasher in the core, as well as the standalone version for slot 0 flashing need some love.

• fix output (messages that overwrite or get overwritten)
• cleanup output
• reduce duplicate code
• perhaps add the possibility to flash different slots than 0 (jtagflash)

[lydon42]

Flashing different slots with jtagflash not yet implemented, but also currently not really needed