Bootloader

Even more updates to the bootloader:

- implement reading fuse bits (so I can easily check EFUSE to see how much flash is left for the program)
- fix address auto-increment (this is apparently not needed by avrdude, but may break something eventually)
- fixes for platforms that are not related to Hexbright (yes, I want to use the same bootloader on other devices)
- cleanup: use macros from standard include files where possible
- cleanup: silence some compiler warnings