nanobsd: Restore boot0cfg functionality

[jlduran]

Summary

The need for this step is fading, now is mostly used to allow the
selection of just the two code partitions in the boot0 boot manager,
instead of the default of allowing all four MBR slices (the other two
being cfg and data, which cannot boot).

Test Plan

Add back NANO_BOOT0CFG functionality.
Before:

Booting from Hard Disk..
1  FreeBSD
2  FreeBSD
3  FreeBSD <-- HERE: this is not a bootable slice, this is /cfg, the default mask 0x3 should prevent it from appearing in the menu
F6  PXE
Boot:  1 .

After:

Booting from Hard Disk..
1  FreeBSD
2  FreeBSD
F6  PXE
Boot:  1 .

Also, should address https://lists.freebsd.org/pipermail/freebsd-embedded/2014-January/002222.html

[imp]

boot0cfg ${NANO_BOOT0CFG} ${NANO_WORLDDIR}/${NANO_BOOTLOADER} doesn't look right. The only bare arg to boot0cfg is the disk, right?
The other one looks good though.

[jlduran]

In D57311#1313282, @imp wrote:

boot0cfg ${NANO_BOOT0CFG} ${NANO_WORLDDIR}/${NANO_BOOTLOADER} doesn't look right. The only bare arg to boot0cfg is the disk, right?

Yes! that is why I created D57310, I'm not sure if it's nonsense.

[imp]

Doh! Of course. Thanks!

[jlduran]

The files /boot/boot0 and /boot/boot0sio have 0444 permissions. chmod before when building unprivileged.