* Break out the boot0 loader selection into a variable - NANO_BOOTLOADER -
so people like me with VGA consoles can override the default (which is
to use boot0sio)
* Put the boot0 configuration options in NANO_BOOT0CFG in case you want
to override the defaults.
* Modify nanobsd.8 to reflect the changes and hint the console default is
serial.
MFC after: 2 weeks
cd4cf11a4d