Add support for 4MHz internal oscillator on all/most parts

[SpenceKonde]

On parts where the lowest supported operating voltage maxes at 4MHz, an internal 4MHz option will be added (it will set fuses for 1MHz, but then change prescaler in init). The x313 parts do not need this as they already have a 4MHz internal oscillator without needing this rigmarole.

[SpenceKonde]

Have I ever said how much I despise the format of boards.txt?

Adding support for this is agonizing - not hard, just incredibly tedious and error prone. A line needs to be added to boards.txt.... for EVERY SINGLE COMBINATION OF PART AND CLOCK SPEED

[SpenceKonde]

Pending travis results, this should be sorted.