Add more background about how the bootloader works

[mithro]

It would be good to expand the section @ http://workshop.fomu.im/en/latest/bootloader.html with more background on how the bootloader works.

It would be good to include;

• [ ] Links to the foboot repository
• [ ] Information on how the multiboot stuff is used.
• [ ] (maybe) Information on the dfu-util stuff