NanoBSD modifications:

• 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