Allow building world into a separate dir (for reuse in multiple images):

Allow building world into a separate dir (for reuse in multiple images):
- buildworld and buildkernel are built into MAKEOBJDIRPREFIX
- installworld and installkernel are performed on NANO_OBJ.

No change of functionality if MAKEOBJDIRPREFIX is not set. If it is sea,t
clean_world deletes NANO_OBJ instead of NANO_WORLDDIR. By starting nanobsd.sh
with the -b option the existing world can be reused to build a new world
reducing time and disk space considerably.

While there:
- Fix two cases where (in comments) MAKEOBJDIRPREFIX should have been
  NANO_DISKIMGDIR.
- Simplify an 'if (not wrong); then true; else action; fi' into
  'if wrong; then action; fi'. 'if ! false; then echo hello; fi' produces hello.

Note: Make sure you use NANO_OBJ were you use MAKEOBJDIRPREFIX now in your
nanobsd.conf files if you want to split out.